Adds an argparse config that, while adding subcommands (`pypi-server run` and `pypi-server update`), retains full commandline backwards compatibility with the existing config parsing logic.
There's a bit of hackery required to do this, so this also issues a warning if using the non-subcommand arguments, allowing us to potentially remove support for the old form in our next next major version bump (i.e. 3.0).
Also adds a `.pyproject.toml` with a black config, and a mypy config block to `setup.cfg`.
`mypy` is now called in `tox`, currently only for `config.py`, because nothing else typechecks successfully.
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* WIP: argparse config
* Complete config
* Test all the config options
* Another test and a note re: being unused
* mypy config, call mypy in tox
* No mypy on pypy
* Fix tox config
* Add venv to black ignore
* fix tox config (again)
* Fix formatting, simplify error handling
* FMT: Run black on changed files
