Require networkx>=2.4 due to Python 3.9.

[tobiasraabe]

Changes

Provide a description and/or bullet points to describe the changes in this PR.

Todo

• Reference issues which can be closed due to this PR with "Closes #x".
• Review whether the documentation needs to be updated.
• Document PR in docs/changes.rst.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.30%. Comparing base (fdc94cb) to head (e948a9c).
Report is 427 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #109   +/-   ##
=======================================
  Coverage   93.30%   93.30%           
=======================================
  Files          62       62           
  Lines        5260     5260           
=======================================
  Hits         4908     4908           
  Misses        352      352           

Flag	Coverage Δ
end_to_end	79.96% <ø> (ø)
integration	41.67% <ø> (ø)
unit	67.22% <ø> (ø)

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