Simplify testing setup + Python 3.12 tests

[dstansby]

I got a bit distracted by wanting to add Python 3.12 testing here 😄 This generally cleans up the testing infastructure by:

• Using a test matrix in GitHub Actions
• Using https://github.com/ymyzk/tox-gh-actions to reduce boilerplate config in tox.ini and the GitHub action configuration

I also noticed fsspec==2023.6.0 was pinned for tests, but newer versions of fsspec break the tests. So I've pinned fsspec in setup.py correctly.