Revert "pin flake8<5.0"

[dbkeitel]

Reverts #424

Let's see if the isort incompatibility has resolved itself.

[codecov]

Codecov Report

Merging #439 (4688b6b) into master (ad6253b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #439   +/-   ##
=======================================
  Coverage   71.80%   71.80%           
=======================================
  Files          11       11           
  Lines        4274     4274           
=======================================
  Hits         3069     3069           
  Misses       1205     1205           

Flag	Coverage Δ
unittests	71.80% <ø> (ø)

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