Add pytest-xdist

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• This PR does not seem to break the templates.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Adjusts the code base to potentially benefit from pytest-xdist
• Removes several old tox-conda configurations (sadly, the library is no longer maintained).

Does this PR introduce a breaking change?

pytest-xdist is now a required development dependency, even if users don't use it.

Other information:

https://pytest-xdist.readthedocs.io/en/stable/

pytest-xdist is a tool for distributing tests across workers. Assuming that the setup is not too heavy, this greatly reduces the amount of time that pytest needs to run tests. The testing code base is now free of potential data races, so there should be no issues.

pytest-xdist is disabled by default as the spin-up time doesn't merit using it yet, but once tests become more numerous, it may be worthwhile to enable it.