fix(gridfs): Adapt code to refactored Mongo driver GridFS implementation

[mdomke]

Closes #903

[codecov]

Codecov Report

Attention: Patch coverage is 96.22642% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.96%. Comparing base (d821f32) to head (f5472ac).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
mongomock/gridfs.py	93.75%	1 Missing ⚠️
tests/test__gridfs.py	97.29%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #905   +/-   ##
========================================
  Coverage    95.95%   95.96%           
========================================
  Files           35       35           
  Lines         9989    10002   +13     
========================================
+ Hits          9585     9598   +13     
  Misses         404      404           

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[P-EB]

Dear @mdomke ,

Do you have plan to release 4.3.X soon or should I consider backporting this PR into Debian, as mongomock is currently not functional there?

Both are fine, I'd just rather use a new release if it's coming soon.

Bests,
PEB