Make tests run in parallel

[abhisrkckl]

The important changes are in ci_tests.yml and tox.ini. The rest are cleanup and formatting.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.93%. Comparing base (dcd3fa8) to head (ee0f5d4).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1913   +/-   ##
=======================================
  Coverage   69.93%   69.93%           
=======================================
  Files         110      110           
  Lines       25705    25705           
  Branches     4056     4056           
=======================================
  Hits        17978    17978           
  Misses       6593     6593           
  Partials     1134     1134           

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[abhisrkckl]

This was tried earlier in #1798, but had to be reverted because the macOS test kept failing. Since then, the macOS test was rewritten to work on M1 chips. So this should work, hopefully. I am not seeing any of the cryptic errors I was seeing with #1798 here.

[dlakaplan]

That all looks pretty simple. Any rough estimate of the speedup?

[abhisrkckl]

All times are in minutes. This includes the overhead of installing packages. Overall, I see ~2x improvement.

Test	Before	After
macos-latest-py313	88	46
oldestdeps	28	14
py313-test-cov	58	30

[dlakaplan]

that's great. I can merge this when you are ready, or deal with #1910 first.

[abhisrkckl]

They are independent. This can be merged.