Installation problem on linux. (Debian and Ubunut)

[TheRealNoSuchNick]

I have had the same problem on two linux boxes (Debian Buster and Ubuntu 16.04) with a clone of master downloaded about 2 weeks ago following the install instructions in README.md.
I was able to install and am just writing this Issue so it can be fixed for future users.

/usr/include/python3.5m is not a symlink. Perhaps this script has already been run.

This happend at the first run (and any others) and in the end I did not need to run this script, but the error message is confusing.

For completeness I ran this after successful install:

lrwxrwxrwx  1 root root   55 Apr 17  2016 numpy -> ../lib/python2.7/dist-packages/numpy/core/include/numpy
drwxr-xr-x  3 root root 4,0K Okt 23 04:29 python2.7
lrwxrwxrwx  1 root root   10 Nov 28  2017 python3.5 -> python3.5m
drwxr-xr-x  2 root root 4,0K Aug 29 03:20 python3.5m

Next Problem is this:

# python3 setup.py install
pygame_sdl2.error is out of date.
Traceback (most recent call last):
  File "setup.py", line 110, in <module>
    cython("pygame_sdl2.error", libs=sdl_libs)
  File "/home/nosuchuser/Projects/Python - SDL/pygame_sdl2-master/setuplib.py", line 245, in cython
    c_fn])
  File "/usr/lib/python3.5/subprocess.py", line 576, in check_call
    retcode = call(*popenargs, **kwargs)
  File "/usr/lib/python3.5/subprocess.py", line 557, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.5/subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1551, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: 'cython'

So I installed the package cython3 using apt. I got the same error from setup.py again, installed the package cython using apt and that fixed it and allowed me to finish the installation. I do not know if both packages are needed, I just thought since I am using python 3 I would need cython3.
The package cython should be added to the apt-get line in README.md.

As I said, I just wrote this to help future users.