[CI] python-version should be a string, not a float

[DimitriPapadopoulos]

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Summary

The underlying issue shows clearly with 3.10, which is interpreted as 3.1.

This does not fix anything that wouldn't currently work. However, for documentation purposes, I feel it is better to use explicit strings. Otherwise, other maintainers may for example trigger this error when updating a version from 3.8 to 3.10.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (75796d5) 63.30% compared to head (1d1b1c2) 63.30%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3522   +/-   ##
=======================================
  Coverage   63.30%   63.30%           
=======================================
  Files         309      309           
  Lines       40879    40879           
  Branches     5383     5383           
=======================================
  Hits        25877    25877           
  Misses      13992    13992           
  Partials     1010     1010           

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[effigies]

Done in #3589