rewrite infrastructure of the project

[pmeier]

This is a complete rewrite of the infrastructure. The most important points are:

• replace tox with doit. Having multiple environments with > 2GB dependencies is not feasible.
• replace isort with usort

[pmeier]

Todo:

• [x] adapt contributing guide
• [x] fix codecov

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@5d21be0). Click here to learn what that means. The diff coverage is n/a.

:exclamation: Current head 3ac5922 differs from pull request most recent head 068fd23. Consider uploading reports for the commit 068fd23 to get more accurate results

@@          Coverage Diff           @@
##             main    #284   +/-   ##
======================================
  Coverage        ?   97.8%           
======================================
  Files           ?      39           
  Lines           ?    1562           
  Branches        ?       0           
======================================
  Hits            ?    1528           
  Misses          ?      34           
  Partials        ?       0           

Flag	Coverage Δ
unit	97.8% <0.0%> (?)

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