Ignore SciPy sparse matrix deprecation warning

[binaydhakal]

Relates to #9353.

SciPy 2.0 nightly warns whenever a sparse matrix class is instantiated, while Distributed promotes warnings to errors during testing. This patch adds a narrowly matched warning filter so the existing serializer test can continue covering all seven sparse matrix classes until SciPy removes them.

The filter only matches the *_matrix is being replaced by *_array. warning; unrelated deprecation warnings remain errors.

Verification

• SciPy 2.0.0.dev0 / NumPy 2.6.0.dev0: 14 serializer cases failed before the change and 14 passed afterward.
• SciPy 1.18.1 / NumPy 2.5.2: all 14 serializer cases passed.
• A matching warning was filtered while an unrelated DeprecationWarning still failed as expected.
• pyproject.toml parses successfully and git diff --check passes.