[ci] Run Cppyy tests in multithreaded mode

[aaronj0]

Should speed up the cppyy jobs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.40%. Comparing base (af760d0) to head (beb05b8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #795   +/-   ##
=======================================
  Coverage   79.40%   79.40%           
=======================================
  Files          11       11           
  Lines        3987     3987           
=======================================
  Hits         3166     3166           
  Misses        821      821           

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[vgvassilev]

I hate these cuts and pastes...

[mcbarton]

To reduce the repetitive pytest options (and future PRs needing to change these options needed to change the ci for all repos), I think we should be using a pytest.toml is cppyy (see https://docs.pytest.org/en/stable/reference/customize.html) and we pass -n auto in that toml file to stop us having to manually set the number of worker processes