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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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