GitHub Action to lint Python code

[cclauss]

Because Travis CI seems to be on (permanent?) vacation...
https://travis-ci.org/github/RobotWebTools/rosbridge_suite

Related to #593 and #600

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.

[amacneil]

These are a lot of steps if someone wants to run lint locally. Can we move it to a configuration file to simplify CI and local linting?

[cclauss]

The four lines from 10 thru 13 should be equivalent on a local machine.

[amacneil]

I don't think this line is necessary? actions/setup-python already gives us the latest version (see output from this on your branch)

[cclauss]

Not always... Watch it over time.

[amacneil]

• I think you need to add C9 here for the cyclomatic complexity check to work? (at least I did on my machine - try setting max-complexity to something lower to see what I mean)
• what do you think about including all F checks (and excluding the failing ones for now) like I did in https://github.com/RobotWebTools/rosbridge_suite/pull/605/files#diff-6951dbb399883798a226c1fb496fdb4183b1ab48865e75eddecf6ceb6cf46442R2-R4 ?

[cclauss]

I think that to F issues should be fixed because PEP8 is clear about wildcard imports being a bad idea.

[amacneil]

sorry I don't understand - can you rephrase?

[amacneil]

Thanks!