Fail to run tests

[bunder]

Host information

$ cat /etc/os-release 
NAME="Arch Linux"
PRETTY_NAME="Arch Linux"
ID=arch
BUILD_ID=rolling
ANSI_COLOR="0;36"
HOME_URL="https://www.archlinux.org/"
DOCUMENTATION_URL="https://wiki.archlinux.org/"
SUPPORT_URL="https://bbs.archlinux.org/"
BUG_REPORT_URL="https://bugs.archlinux.org/"
LOGO=archlinux

$ python --version
Python 3.8.0

$ pip list
Package                       Version    
----------------------------- -----------
absl-py                       0.9.0      
alabaster                     0.7.12     
anytree                       2.7.3      
apipkg                        1.5        
appdirs                       1.4.3      
arrow                         0.15.2     
asn1crypto                    1.2.0      
astor                         0.8.0      
attrs                         19.3.0     
Babel                         2.7.0      
bcrypt                        3.1.7      
Beaker                        1.11.0     
beautifulsoup4                4.8.1      
binwalk                       2.2.0      
btrfsutil                     1.1.1      
CacheControl                  0.12.5     
cached-property               1.5.1      
ceph-volume                   1.0.0      
cephfs                        2.0.0      
cephfs-shell                  0.0.1      
cffi                          1.13.2     
chardet                       3.0.4      
Click                         7.0        
colorama                      0.4.3      
coverage                      4.5.4      
cryptography                  2.8        
cycler                        0.10.0     
Cython                        0.29.14    
decorator                     4.4.1      
distlib                       0.3.0      
distro                        1.4.0      
docker                        4.1.0      
docker-compose                1.25.0     
docker-pycreds                0.4.0      
dockerpty                     0.4.1      
docopt                        0.6.2      
docutils                      0.14       
filelock                      3.0.12     
gast                          0.2.2      
grpcio                        1.25.0     
h5py                          2.10.0     
html5lib                      1.0.1      
httplib2                      0.13.1     
idna                          2.8        
imagesize                     1.1.0      
importlib-metadata            0.23       
iniconfig                     1.0.0      
iotop                         0.6        
isc                           2.0        
jedi                          0.15.1     
Jinja2                        2.10.3     
jsonschema                    3.2.0      
Keras-Applications            1.0.8      
Keras-Preprocessing           1.1.0      
kiwisolver                    1.1.0      
lensfun                       0.3.2      
libvirt-python                5.8.0      
lit                           0.9.0.dev0 
lockfile                      0.12.2     
louis                         3.12.0     
lxml                          4.4.2      
Mako                          1.0.14     
mallard-ducktype              1.0.2      
Markdown                      3.1.1      
MarkupSafe                    1.1.1      
matplotlib                    3.1.1      
meld                          3.20.1     
meson                         0.52.1     
more-itertools                8.0.2      
msgpack                       0.6.2      
namcap                        3.2.10     
nose                          1.3.7      
numpy                         1.17.4     
opt-einsum                    3.1.0      
ordered-set                   3.1.1      
packaging                     19.2       
paramiko                      2.6.0      
parso                         0.5.1      
pep517                        0.6.0      
Pillow                        6.2.1      
pip                           19.2.3     
pluggy                        0.13.1     
ply                           3.11       
progress                      1.5        
protobuf                      3.11.1     
psutil                        5.6.7      
pwquality                     1.4.2      
py                            1.8.0      
pyalpm                        0.9.0      
pyasn1                        0.4.8      
PyAudio                       0.2.11     
pycairo                       1.18.2     
pycparser                     2.19       
pyelftools                    0.25       
Pygments                      2.5.2      
PyGObject                     3.34.0     
PyNaCl                        1.3.0      
pyparsing                     2.4.5      
Pypubsub                      4.0.3      
PyQt5                         5.13.2     
PyQt5-sip                     12.7.0     
PyQtWebEngine                 5.13.2     
pyrsistent                    0.15.6     
python-dateutil               2.8.1      
pytoml                        0.1.21     
pytz                          2019.3     
pyxdg                         0.26       
PyYAML                        5.1.2      
rados                         2.0.0      
rbd                           2.0.0      
requests                      2.22.0     
retrying                      1.3.3      
rgw                           2.0.0      
Send2Trash                    1.5.0      
setuptools                    41.6.0     
sh                            1.12.14    
sip                           4.19.20    
six                           1.13.0     
snowballstemmer               2.0.0      
soupsieve                     1.9.5      
Sphinx                        2.2.1      
sphinx-rtd-theme              0.4.3      
sphinxcontrib-applehelp       1.0.1      
sphinxcontrib-devhelp         1.0.1      
sphinxcontrib-htmlhelp        1.0.2      
sphinxcontrib-jsmath          1.0.1      
sphinxcontrib-qthelp          1.0.2      
sphinxcontrib-serializinghtml 1.1.3      
sphinxcontrib-websupport      1.1.2      
team                          1.0        
tensorflow                    2.1.0rc1   
tensorflow-estimator          2.0.0      
termcolor                     1.1.0      
texttable                     1.6.2      
toml                          0.10.0     
tox                           3.14.2     
tqdm                          4.28.1     
ujson                         1.35       
urllib3                       1.25.7     
virtualenv                    16.1.0     
webencodings                  0.5.1      
websocket-client              0.56.0     
wheel                         0.33.6     
Whoosh                        2.7.4      
wrapt                         1.11.2     
wxPython                      4.0.7.post2
youtube-dl                    2019.11.28 
zipp                          0.6.0

Running nosetests -xdv I get strange error:

test_functional.test_buildhtml_alt ... ERROR

======================================================================
ERROR: test_functional.test_buildhtml_alt
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/aleontiev/projects/sphinx/plantuml/tests/test_functional.py", line 58, in test
    runsphinx(src, builder, confoverrides)
  File "/home/aleontiev/projects/sphinx/plantuml/tests/test_functional.py", line 40, in runsphinx
    app = Sphinx(_srcdir, _fixturedir, _outdir, _outdir, builder,
  File "/usr/lib/python3.8/site-packages/sphinx/application.py", line 232, in __init__
    self.setup_extension(extension)
  File "/usr/lib/python3.8/site-packages/sphinx/application.py", line 395, in setup_extension
    self.registry.load_extension(self, extname)
  File "/usr/lib/python3.8/site-packages/sphinx/registry.py", line 478, in load_extension
    raise ExtensionError(__('Could not import extension %s') % extname, err)
sphinx.errors.ExtensionError: Could not import extension sphinx.builders.epub3 (exception: No module named 'sphinxcontrib.serializinghtml')
-------------------- >> begin captured stdout << ---------------------
Running Sphinx v2.2.1

--------------------- >> end captured stdout << ----------------------

----------------------------------------------------------------------
Ran 1 test in 0.370s

FAILED (errors=1)

sphinxcontrib-serializinghtml is installed and command python -c 'import sphinxcontrib.serializinghtml' executes successfully.

It fails with the same error in tox.