Feature: Linter (flake8) on GitHub Actions

[ymd-h]

For #101

This PR adds a job on GitHub Actions to run flake8 (Linter) at push and pull request.

This utilizes GitHub Super-Linter.

According to the previous PR (#95), some warnings and errors are ignored, which can be configurable at .github/linters/.flake8.

• E129: visually indented line with same indent as next logical line
• E226: missing whitespace around arithmetic operator
• E303: too many blank lines (X)
• E501: line too long (XX > 79 characters)
• W503: line break before binary operator
• W504: line break after binary operator
• E741: do not use variables named ‘l’, ‘O’, or ‘I’

Additionally, a 3rd party copied file tf2rl/misc/prepare_output_dir.py is ignored, too.

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Some code style is also fixed. (58fb730)

[keiohta]

Thanks for the really useful PR!

I would like to confirm the following two things:

1. Does this PR depend on your previous PR (Reduce tf.function retracing #100 )?
2. Does this PR force to edit codes so that they satisfy flake8, or just show messages (warnings)?

[ymd-h]

@keiohta

Does this PR depend on your previous PR (#100 )?

No. These are independent.

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Does this PR force to edit codes so that they satisfy flake8, or just show messages (warnings)?

No. This PR shows linter "test" results for all PRs and pushes.
As you can see, this PR also has "red" results of coverage/coveralls (because of decrease of coverage), however, you can still merge this.
The decision still depends on you.

[keiohta]

@ymd-h Thanks for quick reply. I merged this.