Acknowledge network issues from GitHub

[ambv]

We quite regularly get failures due to sporadic GitHub network issues. This change introduces handling of a new cherry_picker.GitHubException to automatically abort and recover a sane state of the repository.

Previously, the repository was left in a PR_CREATING state, which was not expected by cherry-picker on a re-run, which made miss-islington stall until manually restarted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (aa67b82) to head (ae39a68).

@@            Coverage Diff            @@
##              main      #710   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          357       360    +3     
  Branches        14        14           
=========================================
+ Hits           357       360    +3     

Flag	Coverage Δ
Python_3.11	100.00% <100.00%> (ø)
Python_3.12	100.00% <100.00%> (ø)
Python_3.13	100.00% <100.00%> (ø)

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Files with missing lines	Coverage Δ
miss_islington/delete_branch.py	100.00% <100.00%> (ø)