build: use post-release versioning

[lkstrp]

• Use post-release versioning and do not guess the next version.
• See Version number construction - setuptools scm
• Same behavior as in PyPSA etc.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.07%. Comparing base (4735725) to head (7735a0a).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
+ Coverage   86.00%   86.07%   +0.07%     
==========================================
  Files          17       17              
  Lines        4209     4209              
  Branches      989      989              
==========================================
+ Hits         3620     3623       +3     
+ Misses        430      427       -3     
  Partials      159      159              

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