chore: test to run flake8 on Travis isolated

Author: KingDarBoja

.travis.yml

@@ -9,6 +9,10 @@ python:
   - 3.9-dev
   - pypy
   - pypy3
+matrix:
+  include:
+    - python: 3.6
+      env: TOXENV=flake8
 cache: pip
 install: pip install tox-travis codecov
 script: tox