Travis CI: Upgrade to Python 3.8 and add more flake8 tests (#1304)

.travis.yml

@@ -8,7 +8,7 @@ python-steps: &python-steps
   install: pip install flake8
   script:
     # stop the build if there are Python syntax errors or undefined names
-    - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+    - flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
     # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
     - flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
 
@@ -22,6 +22,5 @@ matrix:
       <<: *node_js-steps
     - python: "2.7"
       <<: *python-steps
-    - python: "3.7"
-      dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
+    - python: "3.8"
       <<: *python-steps