Remove the use of tmpdir and related

[WilliamJamieson]

Description

pytest recommends creating temporary directories with the tmp_path fixture instead of the legacy tmpdir. To check for use of the legacy method one can run with

pytest -p no:legacypath

This PR adds this to he addopts for pytest and then fixes all the issues. In particular the pytest_asdf pluggin blocks all downstream packages from running their tests with this option to detect the legacy temporary directory creations.

Checklist:

• [ ] pre-commit checks ran successfully
• [ ] tests ran successfully
• [ ] for a public change, a changelog entry was added
• [ ] for a public change, documentation was updated
• [ ] for any new features, unit tests were added

[braingram]

Thanks for making these improvements.

The compatibility tests are failing and will need to be updated if we want to use the no:legacypath option. Would you apply similar fixes to the tmpdir usage there?