Copy over topic-* labels to backport PRs

[lysnikolaou]

Closes #640.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (5757871) to head (ddb8c04).

@@            Coverage Diff            @@
##              main      #641   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines         2149      2149           
  Branches       245       245           
=========================================
  Hits          2149      2149           

Flag	Coverage Δ
Python_3.10.14	100.00% <100.00%> (ø)
Python_3.11.9	100.00% <100.00%> (ø)
Python_3.12.3	100.00% <100.00%> (ø)
Python_3.8.18	100.00% <100.00%> (ø)
Python_3.9.19	100.00% <100.00%> (ø)

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Files	Coverage Δ
bedevere/backport.py	100.00% <100.00%> (ø)
tests/test_backport.py	100.00% <100.00%> (ø)

[Mariatta]

Thanks.