fix(ci): bump flake8 to get fix for PyCQA/pyflakes#367; per-file-ignore ext is now redundant; linter back online in CI

.github/workflows/test.yml

@@ -6,8 +6,11 @@ on: [push, pull_request]
 jobs:
   test:
     runs-on: ubuntu-latest
-    container: kmkfw/base
 
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+            python-version: '3.9' # Version range or exact version of a Python version to use, using SemVer's version range syntax
+      - run: python -m pip install --upgrade pipenv wheel
       - run: make test