[maintenance] Upgrade the use of `pip-tools` to leverage its modern features

Feature

$sbj. pip-tools now supports config files (through .pip-tools.toml or pyproject.toml — https://pip-tools.readthedocs.io/en/stable/#configuration). It also supports extracting the build requirements (https://pip-tools.readthedocs.io/en/stable/#maximizing-reproducibility).

Pitch

I saw a couple of pip-tools-managed files in the repo but no documentation on invoking pip-compile or a wrapper script for updating the pins. Having a config file would help with keeping the settings consistent if somebody calls it w/o all the expected args. Here's a config example I tend to use: https://github.com/cherrypy/cheroot/blob/1ff20b1/.pip-tools.toml.

As for the build deps, using one of --build-deps-for [editable|sdist|wheel], --all-build-deps or --only-build-deps can help you get rid of https://github.com/python/mypy/blob/fe15ee6/build-requirements.txt#L1 and https://github.com/python/mypy/blob/fe15ee6/mypy-requirements.txt#L1-L2 which relies on people having to remember to keep things in sync across to copies of deps lists.

cc @hauntsaninja since you were the last to touch build-requirements.txt and @AlexWaygood as you wrote the following in your commit message last changing mypy-requirements.txt:

(Can't say I fully understand why we have to have
this pin in three places :)

Start by reading the pip-tools configuration and build-requirements.txt and mypy-requirements.txt files mentioned in the issue, then compare them with pip-tools' configuration and build-dependency documentation. Identify how dependency pins are currently maintained and how invocation is documented. Done means consistent pip-tools settings, reduced duplicated dependency lists, and documented update commands.