Add single pending review mode

[prince-chrismc]

I needed this for testing pr conan-io/conan-center-index#16144 (comment)

No idea why it was missing but hopefully will be able to catch the problem in the future 🤞

[codecov-commenter]

Codecov Report

Merging #121 (59f63c3) into main (f1c5d0e) will increase coverage by 9.85%.
The diff coverage is 55.69%.

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   44.33%   54.18%   +9.85%     
==========================================
  Files          15       16       +1     
  Lines         609      609              
==========================================
+ Hits          270      330      +60     
+ Misses        316      243      -73     
- Partials       23       36      +13     

Impacted Files	Coverage Δ
pending_review/changes.go	49.27% <49.27%> (ø)
pending_review/pull_request.go	57.00% <100.00%> (+32.73%)	⬆️

... and 2 files with indirect coverage changes

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