GH-49232: [Python] deprecate feather python

[piyushka-ally]

Rationale for this change

Arrow C++ is deprecating the Feather reader/writer (#49231) so we should update the Python functions too. Feather V2 is exactly the Arrow IPC file format, making the separate pyarrow.feather module redundant. Users should migrate to pyarrow.ipc.

What changes are included in this PR?

• Added FutureWarning deprecation warnings to all four public APIs in pyarrow.feather: write_feather(), read_feather(), read_table(), and FeatherDataset
• Extracted _read_table_internal() so that read_feather() calling read_table() internally does not produce double warnings
• Updated FeatherDataset.read_table() to use the internal function for the same reason
• Added .. deprecated:: 24.0.0 directives to all four public API docstrings
• Added module-level pytestmark filter in test_feather.py to suppress warnings in existing tests
• Added 5 new tests verifying deprecation warnings are emitted correctly (including a no-double-warning test)
• Added @pytest.mark.filterwarnings to 5 test functions in test_dataset.py that use feather as a utility
• Updated feather.rst with a deprecation notice and migration guide (including a note that pyarrow.ipc does not compress by default unlike feather.write_feather)
• Updated formats.rst to mark the Feather API section as deprecated

Are these changes tested?

Yes. Five new tests verify that each deprecated function emits exactly one FutureWarning, and existing tests continue to pass with module/function-level warning filters.

Are there any user-facing changes?

Yes — calling pyarrow.feather.write_feather(), read_feather(), read_table(), or FeatherDataset() now emits a FutureWarning directing users to pyarrow.ipc equivalents. Existing functionality is unchanged.

• GitHub Issue: [Python] Deprecate Feather reader and writer #49232

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[AlenkaF]

Thanks for the PR @piyushka-ally!
I think it looks good and I just have two minor comments

Also, it might be worth syncing with Arrow C++ and other bindings so that we could get the deprecation in the same release. @pitrou is there an open PR for C++?

[piyushka-ally]

Thanks for the PR @piyushka-ally! I think it looks good and I just have two minor comments

Also, it might be worth syncing with Arrow C++ and other bindings so that we could get the deprecation in the same release. @pitrou is there an open PR for C++?

The R deprecation PR is already open at #49276. I don't see a C++ PR for #49231 yet. I'll watch for it. Happy to rebase/adjust timing if needed to align with the same release.

[AlenkaF]

Thanks for a quick update!
I missed something, added one more comment.

[AlenkaF]

I am not sure if pandas mark is needed here?

[piyushka-ally]

The @pytest.mark.pandas is there because read_feather calls .to_pandas() internally, so the test would fail in environments without pandas installed. write_feather and read_table don't need it, but read_feather does. Should I split this back into two tests, one without the pandas mark for write_feather/read_table, and a separate pandas-marked one for read_feather? Or would you prefer a different approach?