- 2.7 was EOL'd in 2020
- 3.6 was EOL'd in 2021
- 3.7 will be EOL'd in June 2023

Even if a 1.0 is cut before June it doesn't seem likely that it'll
matter a lot.

3.8 doesn't bring a lot that's likely to be useful as far as I know,
candidates I can think of are:

- positional-only parameters
- typed dicts and literals, to add typing to the legacy API

While at it, remove the now unnecessary future imports, as well as the
module-level docstrings (they're not very helpful) and the per-file
license headers (they don't seem useful).

PEP 517
=======

Don't migrate away from setuptools, but move "static" packaging
content from `setup.py` to `pyproject.toml` and `setup.cfg`. Building
should now be performed using
[`build`](https://pypa-build.readthedocs.io/en/stable/), as direct
invocations of `setup.py` have been deprecated for a while.

Also remove the legacy `__author__` strictures from the source files,
as `pyproject.toml` supports an authors section which is more suitable.

Codegen Modernization
=====================

The modernization via command overrides and self-invocation still
generated `setup.py` warnings.

Migrate the "invoke commands in commands" method to codegen
sub-commands in `build`, it looks like this suffices for what we
need. This reuses the existing code, though modernizes it
significantly thanks to the previous commit.

This is strongly inspired by @abravalheri's comments on codegen in
setuptools discussions e.g. pypa/setuptools#3180 and
pypa/setuptools#3762, those comments were very helpful in getting a
better understanding of the sub_commands system (also @jaraco's
comments but those were a touch terse so can't say I really got it
until sumbling on @abravalheri's).

Leverages the setuptools subcommands protocol to support editable
wheels, as that might be useful. Don't use editable wheels for testing
though, as I don't see the point.

Note: not sure the git submodule stuff is useful during the codegen,
so currently commented it out.

sdists
======

I'm not entirely clear what sdists should really be about, adding
codegen to sdist using subcommands seems not necessarily trivial, so
for now make them basically an export:

- remove `_regexes.py` from sdist
- add uap-core to the manifest, and thus the sdist, at least
  `regexes.yaml`

Currently the test files (both python and yaml) are also included, but
it's not clear that they *should* be. That seems like [a whole
debate](https://discuss.python.org/t/should-sdists-include-docs-and-tests/14578/117),

One strong argument (to me) in favor of a sdist maximalism is that it
matches the wheels and more official releases, but in that case the
sdist should probably be better crafted than what I threw together.

Tox
===

Switch tox to an explicit `pip install` as its builtin handling for
installing packages (whether via sdist or develop) does not seem
compatible with PEP-517. Need to think about moving to tox 4, which
apparently has native support for PEP 517 packages. Cf ua-parser#157.

Also remove the `docs` tox env, it hasn't been a thing since
8273477 but I forgot to remove it
from the envlist.

- remove old compile step
- update python-action to v4 and "primary" python to 3.11
- produce sdist and wheel just once
- ensure we're testing after installing from wheel, sdist, and source