Update GitHub Actions configuration

[leouieda]

Adopt some of the newer configuration from our other projects. Use mamba instead of conda to speed up runs. Test only on the Python version end members. Organize the requirements files and separate dependency types. Run the PyPI building and archive checking in PRs but only deploy on pushes and releases. Add the license check to the style checks.

Also updates the setuptools_scm configuration to generate a source file at build time so it's not a run-time dependency anymore. Had to make some changes to make sure the other parts used the correct version information and avoiding circular imports.

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

The failing check is codecov complaining that there is a bit more code in this version 🙂

[leouieda]

Merging this in since it's holding things back a bit and it's not breaking anything.