Migrate to `extend-exclude` in flake8 config
Migrate to `extend-exclude` in flake8 config

This makes sure that flake8's defaults are in use. Specifically, it
excludes `.tox/` dir that is known to contain a lot of files in nested
folders that are not supposed to be linted.

Refs:
* https://github.com/pypa/setuptools/issues/2501#issuecomment-749144396
* https://github.com/pypa/setuptools/pull/2486/files#r546877674
* https://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-extend-exclude
* https://flake8.pycqa.org/en/latest/user/options.html#cmdoption-flake8-exclude

pypa/setuptools,pypa/setuptools,pypa/setuptools