GitHub Action to lint Python code with flake8

[cclauss]

Summary

Results: https://github.com/cclauss/pycord/actions
Failing tests resolved in #75 and #76

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.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new tests).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[cclauss]

@hugovk Can I please get your advise on why flake8 is raising F722 and F821 errors on examples/app_commands/slash_options.py? #76

Mypy (#81) suggests:

examples/app_commands/slash_options.py:13: error: Invalid type comment or annotation
examples/app_commands/slash_options.py:13: note: Suggestion: use Option[...] instead of Option(...)
examples/app_commands/slash_options.py:14: error: Invalid type comment or annotation
examples/app_commands/slash_options.py:14: note: Suggestion: use Option[...] instead of Option(...)
examples/app_commands/slash_options.py:15: error: Invalid type comment or annotation
examples/app_commands/slash_options.py:15: note: Suggestion: use Option[...] instead of Option(...)

[hugovk]

Hi! I'm not so familiar with type annotations, https://stackoverflow.com/a/64917499/724176 shows some annotations which are incompatible with PEP 484, which pyflakes uses. The suggestion is to come up with PEP 484-compatible annotations, or to ignore the errors in Flake8.