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:
* pypa#2501 (comment)
* 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

.flake8

@@ -1,6 +1,6 @@
 [flake8]
 max-line-length = 88
-exclude =
+extend-exclude =
 	setuptools/_vendor
 	pkg_resources/_vendor
 ignore =