GitHub Action to lint Python code

[cclauss]

Output: https://github.com/cclauss/DBUtils/actions

https://flake8.pycqa.org/en/latest/user/error-codes.html

On the flake8 test selection, this PR does not focus on "style violations" (the majority of flake8 error codes that psf/black can autocorrect). Instead, these tests are focus on runtime safety and correctness:

• E9 tests are about Python syntax errors usually raised because flake8 can not build an Abstract Syntax Tree (AST). Often these issues are a sign of unused code or code that has not been ported to Python 3. These would be compile-time errors in a compiled language but in a dynamic language like Python, they result in the script halting/crashing on the user.
• F63 tests are usually about the confusion between identity and equality in Python. Use ==/!= to compare str, bytes, and int literals is the classic case. These are areas where a == b is True but a is b is False (or vice versa). Python >= 3.8 will raise SyntaxWarnings on these instances.
• F7 tests logic errors and syntax errors in type hints
• F82 tests are almost always undefined names which are usually a sign of a typo, missing imports, or code that has not been ported to Python 3. These also would be compile-time errors in a compiled language but in Python, a NameError is raised which will halt/crash the script on the user.

[Cito]

Thank you for this proposal. We already check with flake8 using Travis, but I wanted to migrate to GitHub Actions anyway, see #30. Not sure if all of these checks make sense for DBUtils (I see you already deactivated most of them with || true), but I will certainly consider your PR. Maybe we should also run the checks with tox so that the same tests can be executed locally.

[cclauss]

https://docs.github.com/en/actions/guides/building-and-testing-python#running-tests-with-tox

Or

https://github.com/tox-dev/gh-action-tox

[cclauss]

Closed in favor of #31