Test on more platforms, more Python versions, more MNE versions

[hoechenberger]

While we should keep the current CircleCI-based test setup with the relatively large number of test datasets, I believe we should run some basic tests (with a single, smaller dataset) on more platforms, as the current tests all run on the exact same platform.

My proposal is to add additional tests via GHA to run on:

• ubuntu-latest, macos-latest, windows-latest, macos-14 (Apple Silicon)
• MNE-Python main
• and potentially different versions of Python

@larsoner WDYT?

[larsoner]

Sure, we already have rerun timing / caching tests on GHA using some of our smaller datasets. It shouldn't be too hard to adapt them to related different platforms or versions of MNE. Feel free to try it if you want