Update setup methods and Python version requirements

[mattpitkin]

In this MR, I have:

• dropped support for Python 3.7, so the minimum support version is Python 3.8.
• remove some old Python 2 compatibility workarounds that are no longer necessary
• moved most setup information from setup.py into pyproject.toml (including building with NumPy v2.0.0, which allows NumPy v2.0.0 compatibility, while still supporting running with earlier versions.)
• switched from manual versioning to instead use setuptools_scm, i.e., versions are determined by git tags.
• some Mac OS GitHub action CI fixes from #67 and others that now fix the CI.

@vallis - are you happy for these changes to be made?

If/when merged, I'd propose creating a new v3.5.0 release.