test: Silenced PyMuPDF warnings

[fg-mindee]

This PR silences the deprecation warnings from PyMuPDF.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #740 (688b5dc) into main (808081f) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #740      +/-   ##
==========================================
+ Coverage   96.05%   96.24%   +0.18%     
==========================================
  Files         129      129              
  Lines        4794     4794              
==========================================
+ Hits         4605     4614       +9     
+ Misses        189      180       -9     

Flag	Coverage Δ
unittests	96.24% <100.00%> (+0.18%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
doctr/datasets/loader.py	100.00% <100.00%> (ø)
doctr/models/builder.py	99.12% <0.00%> (+2.63%)	⬆️
doctr/models/detection/linknet/base.py	91.08% <0.00%> (+3.96%)	⬆️
doctr/models/recognition/predictor/tensorflow.py	97.05% <0.00%> (+5.88%)	⬆️

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[charlesmindee]

thanks!