Update development toolchain

[dave-shawley]

This PR replaces the setuptools based pipeline with a pyproject file using hatch as the build backend. I chose hatch since it was adopted by the Python Packaging Authority and because it is nicely interoperable with the common "bring your own virtualenv" workflow. Personally, I prefer using hatch directly. The updated contributing file describes how to use it directly as well as using a freestanding virtual env.

• replace setup.cfg and setup.py with pyproject.toml
• add a pre-commit file to encourage style updates before submitting a PR
• add a readthedocs configuration file
• drop explicit testing (not support) of Python 3.7
• drop support for 3.5 and 3.6

I don't plan on releasing this version to pypi. I plan on removing the various deprecated things before releasing a 3.0 to pypi.