Switch to use pyproject.toml and fix typing issue

[CoolCat467]

In this pull request, we switch the project configuration to use pyproject.toml and fix a syntax error in the typing stubs I encountered using mypy on a project that uses this project.

[Pasu4]

It may be better to split these two changes into separate PRs, so in case switching to pyproject takes further discussion or is not wanted, the fix for the syntax error can still go through (or go through faster than otherwise).

[CoolCat467]

That's a good idea, will split this into two PRs, leaving this one for pyproject.toml, and will make another one for the syntax error that is immediately impacting us.

[rnag]

@CoolCat467 Thanks for working on this PR! This change breaks CI because tox is now reading setup.cfg instead of pyproject.toml.
Since the PR removed tox.ini, can you also update the GitHub Actions job to explicitly install tox 4.x (and the gh plugin if needed) so tool.tox.legacy_tox_ini is picked up correctly?

[CoolCat467]

Removed setup.cfg and moved all configuration it contained to pyproject.toml

[rnag]

@CoolCat467 Thanks! It looks like there are some last-minute merge conflicts but they should be trivial - setup.py was removed for example. Are you able to take a look when you have a chance?

I would normally try to resolve them, but I'm seeing it grayed out with an error:

These conflicts are too complex to resolve in the web editor.

[CoolCat467]

Sure thing

[rnag]

@CoolCat467 LGTM! Thanks for your contribution on this.

[rnag]

@CoolCat467 These changes should be live as of 0.36.3. I've also credited you in the release notes. Thanks for your help on this!