[MRG] Do not use pytest.warns(None)

[mrbean-bremen]

• avoids deprecation warnings from pytest
• fixes Fix deprecated pytest.warns(None) #1621

I did not add release notes, I think they are not needed for this.

Tasks

• Fix or feature added
• Code typed and mypy shows no errors

[codecov]

Codecov Report

Merging #1623 (27e58a6) into master (728c8ac) will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1623      +/-   ##
==========================================
+ Coverage   97.57%   97.67%   +0.10%     
==========================================
  Files          66       66              
  Lines       10660    10660              
==========================================
+ Hits        10401    10412      +11     
+ Misses        259      248      -11     

Impacted Files	Coverage Δ
pydicom/valuerep.py	99.53% <0.00%> (+0.15%)	⬆️
pydicom/filewriter.py	98.06% <0.00%> (+0.24%)	⬆️
pydicom/filereader.py	94.62% <0.00%> (+0.25%)	⬆️
pydicom/uid.py	100.00% <0.00%> (+1.09%)	⬆️
pydicom/filebase.py	99.19% <0.00%> (+1.61%)	⬆️

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