Change build backend from flit to hatchling

[jaimergp]

Description

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

I forgot to update this attribute in the last release, so just fixing it now so it's clear in the future. Internally, conda-libmamba-solver.__version__ will report 22.8.1 til this is released. To be clear, conda list does report the correct version. This now uses hatchling plus a setuptools-scm plugin, hatch-vcs, as the build backend.

[jezdez]

@jaimergp We could switch to setuptools_scm via https://pypi.org/project/flit-scm/ or (while looking at pypa/flit#257) switch to PyPA's hatch instead that has setuptools_scm support?

[jaimergp]

We chose flit because back then it was the only solution that provided editable installs with a pyprojec.toml-only setup. If that has changed, then we can revisit that option. flit-scm looks simple enough to not break easily, despite being a "minor" project if compared with all the other backends.

Other alternatives include adding an extra step to the Release issue template, and manually update the version attribute as part of the "Prepare release" PR.

[jaimergp]

I tried all options locally and hatch-vcs fits the bill just fine!