Fix sphinx config

[dweindl]

Convert path-like _StrPath object to str. Changed in sphinx>=8.

Fixes #1482.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.90%. Comparing base (b091d46) to head (48502f5).

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1483      +/-   ##
===========================================
- Coverage    82.93%   82.90%   -0.03%     
===========================================
  Files          163      163              
  Lines        13786    13786              
===========================================
- Hits         11433    11429       -4     
- Misses        2353     2357       +4     

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[PaulJonasJost]

Very much thanks for this :)