Implement $pop

[oorijsonar]

No description provided.

[codecov]

Codecov Report

Merging #602 into develop will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #602      +/-   ##
===========================================
+ Coverage    94.69%   95.04%   +0.35%     
===========================================
  Files           16       17       +1     
  Lines         3187     3313     +126     
===========================================
+ Hits          3018     3149     +131     
+ Misses         169      164       -5     

Impacted Files	Coverage Δ
mongomock/collection.py	92.16% <100.00%> (+0.48%)	⬆️
mongomock/filtering.py	98.56% <0.00%> (-0.34%)	⬇️
mongomock/read_concern.py	100.00% <0.00%> (ø)
mongomock/mongo_client.py	97.08% <0.00%> (+0.11%)	⬆️
mongomock/aggregate.py	97.41% <0.00%> (+0.53%)	⬆️

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[pcorpet]

Please make sure to satisfy the CI by finalizing the test coverage.