Add automatic code formatting to Requests

[nateprewitt]

Overview

This is a draft PR for adding standardized code formatting to Requests. Now that we intend to drop Python 2 support, we have the opportunity to clean up multiple conflicting code styles that have crept into Requests over the years.

This PR will introduce a few standard changes:

• Integration of black for code formatting, bringing Requests in-line with what has largely become the standard.
• Integration of isort to ensure our imports are automatically grouped and cleaned.
• Integration of pyupgrade which will automatically pull most of the code base to Python 3.7 syntax.
• Addition of a .pre-commit-config.yaml to orchestrate the above tools.

TODO

• Rebase on to Remove support for Python 2.7, 3.6 and Pypy3.6 #6091 before merge
• Resolve formatting issues with server response line wrapping in tests/test_lowlevel.py
• Add .git-blame-ignore-revs file (Add .git-blame-ignore-revs #6116)
• Fix inconsistent f-string upgrades
• Enable and fix flake8 issues

[verhovsky]

https://github.com/psf/requests/blob/main/docs/dev/contributing.rst#kenneth-reitzs-code-style is no longer necessary if you're formatting the code

[sethmlarson]