Updating from setup.py to pyproject.toml

[MrHemlock]

This PR serves to to update from the traditional setup.py model to the more modern pyproject.toml. This makes PDM a requirement when building and publishing the project. The proper integrations have been (at least from what we can see) adjusted in AppVeyor and should be ready to go. Should for sure be tested before pushing it through.

[CLAassistant]

All committers have signed the CLA.

[FeodorFitsner]

Thanks for your PR! Would it make sense to update the requirement for Python to >=3.8? I'm not sure why I put 3.7 in the first place, but even PDM suggests 3.8 by default. Is there any benefit of supporting 3.7?

[MrHemlock]

>=3.7 actually works perfectly. We're going to be using the annotations module from __future__ which will allow us to integrate modern type hinting as far back as 3.7. At present, there isn't anything breaking that would prevent us from using 3.7, so no change needed.