Remove automerge label if not added by core dev

[Mariatta]

This also adds the "Automerge-triggered-by: @{coredev}" message at the end of the PR body.

Closes python/core-workflow#340
Closes #155

[black-out]

🐍🌚🤖 @Mariatta, I've formatted these files using black:

• miss_islington/status_change.py
• miss_islington/util.py
• tests/test_status_change.py
  (I'm a bot 🤖)

[codecov]

Codecov Report

Merging #253 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   95.94%   96.16%   +0.22%     
==========================================
  Files           9        9              
  Lines         813      860      +47     
  Branches       49       52       +3     
==========================================
+ Hits          780      827      +47     
  Misses         33       33

Impacted Files	Coverage Δ
miss_islington/status_change.py	100% <100%> (ø)	⬆️
tests/test_status_change.py	100% <100%> (ø)	⬆️
miss_islington/util.py	100% <100%> (ø)	⬆️

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