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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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