Use pytest-xdist

[mdbenito]

Description

This PR makes our lives nicer by using pytest-xdist. We automagically set the number of pytest workers to avoid oversubscription by looking at num_workers (the number of parallel processes started within a test using a parallel backend)

Changes

• Required pytest-xdist
• Made num_workers not a fixture
• Used -n auto in calls to pytest inside tox.ini

Checklist

• [ ] ~Wrote Unit tests (if necessary)~
• [ ] ~Updated Documentation (if necessary)~
• [x] Updated Changelog
• [ ] ~If notebooks were added/changed, added boilerplate cells are tagged with "tags": ["hide"] or "tags": ["hide-input"]~

[AnesBenmerzoug]

This doesn't seem to improve the test runtime on CI. Is that expected?

[mdbenito]

This doesn't seem to improve the test runtime on CI. Is that expected?

For the record: yes, this only improves tests on machines with > 4 cores.