Don't merge PR unless it has "awaiting merge" label

[Mariatta]

Change the behavior for when deciding when miss-islington
should merge the PR.

Previously it checks if the PR has one core dev who approved the PR.
But that is unreliable because the review may be dismissed later,
or another core dev might decide not to backport the PR anymore.

The "awaiting merge" label is a better indicator that the PR
is ready for merge.

Adjust tests.

Fixes #62

[codecov]

Codecov Report

Merging #112 into master will increase coverage by 1.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
+ Coverage    91.1%   92.22%   +1.11%     
==========================================
  Files           9        9              
  Lines         551      566      +15     
  Branches       40       40              
==========================================
+ Hits          502      522      +20     
+ Misses         49       44       -5

Impacted Files	Coverage Δ
tests/test_delete_branch.py	100% <ø> (+4.44%)	⬆️
miss_islington/status_change.py	100% <100%> (ø)	⬆️
tests/test_util.py	100% <100%> (+4.22%)	⬆️
miss_islington/util.py	75% <100%> (+2.11%)	⬆️
tests/test_status_change.py	100% <100%> (ø)	⬆️

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[miss-islington]

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

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