CI: only collect coverage data for one Python version

[superlopuh]

It slows down all the tests, even though we only upload the coverage data for 3.10 currently. This will help test both branches of #3953.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.04%. Comparing base (e294346) to head (4450a63).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4051   +/-   ##
=======================================
  Coverage   89.04%   89.04%           
=======================================
  Files         317      317           
  Lines       43338    43338           
  Branches     5403     5403           
=======================================
  Hits        38590    38590           
  Misses       3406     3406           
  Partials     1342     1342           

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