To keep build-compatibility with Python 3.6, I've switched the build
backend from `setuptools` to `hatchling`. `setuptools` only supports
`pyproject.toml` from version 61 onwards, which itself isn't compatible
with Python 3.6. This does not mean we have to fully use Hatch as we can
still create build using `python -m build`. Hatchling automatically
includes all non-gitignored files, so the `MANIFEST.in` file has been
removed.

The latest incarnation of bumpversion is bump-my-version, which
considers .cfg configuration "deprecated" and wants it moved
to TOML, so here we go. We may as well move pytest config at
the same time as pytest has supported it for ages. We can drop
the "universal wheel" thing as that was about supporting Python
2 and 3 in the same wheel, and we don't support Python 2 any
more.

flake8 does not support pyproject.toml and it doesn't look like
you can specify aliases for setup.py there either, so we'll leave
those two in setup.cfg for now. Maybe in future we can get rid
of them.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

This fails at least when run via tox on current Fedora, flake8
tries to parse the # as a directive and chokes on it.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

Reformat and add max-line-length constraint to flake8 config.

This fixes tests not being added to the source package.

Bumpversion cannot update the release date and trying to remember to
update it manually won't work.