nix integration: use python3 instead of python310

[nagisa]

Problem

python310 was dropped upstream in NixOS/nixpkgs#490538

Summary of Changes

use plain python3 which should mostly work until python releases a 4.0.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.0%. Comparing base (f3f9405) to head (d2c4682).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10847   +/-   ##
=======================================
  Coverage    83.0%    83.0%           
=======================================
  Files         835      835           
  Lines      315854   315943   +89     
=======================================
+ Hits       262220   262304   +84     
- Misses      53634    53639    +5     

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[joncinque]

Makes sense, thanks!