Prep release 5.0.12

[bblommers]

Fixes release process:

• Commit version number changes before creating a release
• This ensures that the PyPi release has the exact same changes as the commit it references
• Include setup.cfg, as the version nr in that file hadn't been updated in some time

Also adds pytest markers to get rid of the dozens of warnings when running tests.
These markers were already defined in setup.cfg - not sure why they weren't picked up before though.

Includes some tiny code/test improvements, but no logic changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.40%. Comparing base (0f25f2f) to head (817e7c9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7929      +/-   ##
==========================================
- Coverage   94.40%   94.40%   -0.01%     
==========================================
  Files        1109     1109              
  Lines       95214    95209       -5     
==========================================
- Hits        89886    89881       -5     
  Misses       5328     5328              

Flag	Coverage Δ
servertests	29.12% <ø> (+<0.01%)	⬆️
unittests	94.37% <ø> (-0.01%)	⬇️

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