Use exact Python versions for CI

[puddly]

See zigpy/workflows#28

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.69%. Comparing base (a1897cb) to head (15cc878).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #418   +/-   ##
=======================================
  Coverage   96.69%   96.69%           
=======================================
  Files          61       61           
  Lines        9863     9863           
=======================================
  Hits         9537     9537           
  Misses        326      326           

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