depr(python): Rename fmt param to f_string in inspect()

[etiennebacher]

Fixes #19442

It was suggested in the issue to rename the fmt param and f_string already exists in pl.format(). This also adds documentation for this parameter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.72%. Comparing base (73cb2a2) to head (cfddaf3).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20748      +/-   ##
==========================================
+ Coverage   79.70%   79.72%   +0.01%     
==========================================
  Files        1561     1561              
  Lines      221930   221910      -20     
  Branches     2530     2530              
==========================================
+ Hits       176892   176910      +18     
+ Misses      44456    44418      -38     
  Partials      582      582              

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