MAINT: cleaning pytest cache on first run

[bsipocz]

to make tox envs locally reusable

resolves #3416

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.51%. Comparing base (c285462) to head (08432fa).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3417   +/-   ##
=======================================
  Coverage   70.51%   70.51%           
=======================================
  Files         232      232           
  Lines       19997    19997           
=======================================
  Hits        14100    14100           
  Misses       5897     5897           

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