Improvements to the build process

[marvinpfoertner]

• read the package version from the most recent git tag
• reactivate triggers for testing and linting CI (these broke because of the master -> main renaming)
• make Cython builds optional, even in PEP517 builds
• fix warnings in setup.cfg
• fix Cython build warning

[codecov-commenter]

Codecov Report

Merging #15 (6d0ad18) into main (bb8c095) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   95.97%   96.00%   +0.02%     
==========================================
  Files           7        7              
  Lines         174      175       +1     
==========================================
+ Hits          167      168       +1     
  Misses          7        7              

Impacted Files	Coverage Δ
src/cholupdates/__init__.py	100.00% <100.00%> (ø)

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