GitHub Action to lint Python code

[cclauss]

Results: https://github.com/cclauss/VTK/actions

$ flake8 . --count --select=E9,F63,F7 --show-source --statistics

[ ... ]
15    E999 SyntaxError: invalid syntax
15

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 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.

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[banesullivan]

FYI, this repository is a mirror of https://gitlab.kitware.com/vtk/vtk and PRs are not frequently checked here. I'd recommend reopening a PR on GitLab: https://gitlab.kitware.com/vtk/vtk

[cclauss]

I have tried twice but the CI over there is so complex that it is impossible to get these simple tests to run.
https://gitlab.kitware.com/vtk/vtk/-/merge_requests?scope=all&author_username=cclauss

[banesullivan]

I just found your https://gitlab.kitware.com/vtk/vtk/-/merge_requests/8400

I'm going to add this to my list and see if I can help down the road... Might be a while though as the VTK python code may undergo bigger changes as proposed here: https://discourse.vtk.org/t/more-pythonic-interface-closer-to-numpy/7905