Switch to using `pyproject.toml`

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
• "closes #XXXX" is in the body of the MR description to link the related issue, if applicable.

I switched to predominantly using pyproject.toml. A few things:

1. I haven't yet translated the cmdclass from setup.py to pyproject.toml, so I left that in setup.py for now. Looks like you can do so as described here, but my first attempt failed.
2. Is it really necessary to be shipping the ase/test/testdata with the package by default? Is the ability to run tests without cloning the repo a greater benefit than increased package size?