Reducing the number of dependencies

[sobolevn]

Currently our linter has lots of dependencies.
There are several types of them:

1. Fundamental, like flake8. Without these packages our linter won't work at all
2. Important, like typing_extensions, pygments, pep8-naming, flake8-bugbear, flake8-isort, and several others. Which give awesome features and increases the final quality be a large margin
3. Single-featurish and complex, like flake8-commas, flake8-quotes, flake8-builtins. Which give us an awesome single feature, but is too complex to recreate
4. Single-featurish and simple, like flake8-print, flake8-debugger, flake8-annotations-complexity, cognitive_complexity, flake8-pep3101. Which give us an awesome single feature, but is fairly simple to recreate

I suggest to drop all possible group 4 dependencies and recreate their logic inside the core product. Why?

Because they are harder to manage in the long run. See #1138 for more details.

Downsides:

• error code change for our users
• extra code to maintain
• more tests to write
• unpredictable effects on performance

I hope this makes sense.

[sobolevn]

• flake8-executable can be dropped, here's the full implementation: https://github.com/xuhdev/flake8-executable/blob/abf67771703b2f06f02a3fedb1dd97299583a4c3/flake8_executable/__init__.py#L92-L128
• flake8-annotation-complexity can be dropped, here's the full implementation: https://github.com/best-doctor/flake8-annotations-complexity/blob/master/flake8_annotations_complexity/ast_helpres.py#L5-L34 We also need to copy the configuration for this one
• flake8-print can be dropped, here's the full implementation https://github.com/JBKahn/flake8-print/blob/master/flake8_print.py Moreover, we can just add print and pprint functions the blacklist and change nothing else
• flake-pep3101 can be dropped, here's the full implementation: https://github.com/gforcada/flake8-pep3101/blob/master/flake8_pep3101.py
• flake8-coding can be dropped, here's the full implementation: https://github.com/tk0miya/flake8-coding/blob/master/flake8_coding.py#L52-L73 We also need to copy the configuration for this one
• cognitive_complexity can be dropped, here's the full implementation: https://github.com/Melevir/cognitive_complexity/blob/master/cognitive_complexity/utils/ast.py

Can be dropped for python3.8:

• typing_extensions (we still require it for python<3.8)

Can be dropped after libcst integration:

• flake8-broken-line

Can be dropped after python3.9:

• astor

[sobolevn]

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