Fix unittest warning handling with pytest 8.0

[guarin]

Changes

• Use warnings.catch_warning instead of pytest.warns(None) as the latter was deprecated in pytest 8.0

This fixes a bug where our unit tests recently started failing due to the deprecation.
Example of failing test: https://github.com/lightly-ai/lightly/actions/runs/7708499394/job/21007710296

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6bd7553) 84.38% compared to head (d34c781) 84.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1489      +/-   ##
==========================================
+ Coverage   84.38%   84.40%   +0.02%     
==========================================
  Files         139      139              
  Lines        5756     5777      +21     
==========================================
+ Hits         4857     4876      +19     
- Misses        899      901       +2     

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