Fix Automerge functionality

[Mariatta]

Look up conclusions from check runs in addition to status checks.

Ensure all check runs have completed.
Ensure there is no "failure" or "timed_out" result from the check runs

Fixes #375

[black-out]

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

• tests/test_status_change.py
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[codecov]

Codecov Report

Merging #382 into main will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #382      +/-   ##
==========================================
+ Coverage   96.42%   96.51%   +0.09%     
==========================================
  Files           9        9              
  Lines         924      948      +24     
  Branches       55       58       +3     
==========================================
+ Hits          891      915      +24     
  Misses         33       33              

Impacted Files	Coverage Δ
miss_islington/status_change.py	100.00% <100.00%> (ø)
miss_islington/util.py	100.00% <100.00%> (ø)
tests/test_status_change.py	100.00% <100.00%> (ø)