Fix Python 3.12 build

[offa]

Fixes the Python 3.12 CI build (see #1262).

Remarks:

The flake8 version used in the CI build is quite old, but an upgrade is blocked by flake8-no-fstring (needs flake8<4). An update to the latest version isn't possible as flake8 6.x does no longer support Python 3.6 and 3.7.

Are importlib-metadata downgrade and flake8-no-fstring still necessary? The later should be fixed by recent flake8 versions.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (b85ac5f) 34.42% compared to head (dc754fe) 34.42%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1263   +/-   ##
=======================================
  Coverage   34.42%   34.42%           
=======================================
  Files          44       44           
  Lines        8116     8116           
  Branches     1123     1123           
=======================================
  Hits         2794     2794           
  Misses       5208     5208           
  Partials      114      114           

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