[Python] Fix deprecation warnings in the tests with numpy/pandas nightly

[jorisvandenbossche]

Some of the warnings I am seeing (from https://github.com/ursacomputing/crossbow/actions/runs/34548523730/job/103106277981):

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_array.py::test_array_from_timestamp_with_generic_unit
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_array.py:2682: DeprecationWarning: The 'generic' unit for NumPy timedelta is deprecated, and will raise an error in the future. This includes implicit conversion of bare integers (e.g. `+ 1`). Please use a specific unit instead.
    n = np.datetime64('NaT')

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_array.py::test_array_from_numpy_timedelta_incorrect_unit
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_array.py:2723: DeprecationWarning: The 'generic' unit for NumPy timedelta is deprecated, and will raise an error in the future. This includes implicit conversion of bare integers (e.g. `+ 1`). Please use a specific unit instead.
    td = np.timedelta64(1)

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py:2648: Pandas4Warning: DatetimeIndex.dayofweek is deprecated and will be removed in a future version. Use DatetimeIndex.day_of_week instead.
    dayofweek = ts.dt.dayofweek.astype("int64")

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_extract_datetime_components
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py:2649: Pandas4Warning: DatetimeIndex.dayofyear is deprecated and will be removed in a future version. Use DatetimeIndex.day_of_year instead.
    dayofyear = ts.dt.dayofyear.astype("int64")

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_round_temporal[nanosecond]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_round_temporal[microsecond]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_round_temporal[millisecond]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_round_temporal[second]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_round_temporal[minute]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_round_temporal[hour]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py::test_round_temporal[day]
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_compute.py:2972: UserWarning: The 'unit' keyword is only used when the Timestamp input is an integer or float, not str. To specify the storage unit of the output use `ts.as_unit(unit)`
    ts = pd.Series([pd.Timestamp(x, unit="ns") for x in timestamps])

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dataset.py::test_make_fragment
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dataset.py:243: Pandas4Warning: DatetimeIndex.dayofweek is deprecated and will be removed in a future version. Use DatetimeIndex.day_of_week instead.
    for part, chunk in df_b.groupby([df_b.date.dt.dayofweek, df_b.color]):

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py::test_dlpack_legacy_capsule_immutable_tensor[None]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py::test_dlpack_legacy_capsule_immutable_tensor[max_version1]
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py:78: DeprecationWarning: Exporting an unversioned DLPack capsule is deprecated, pass max_version=(1, 0) or higher.
    return f(*args, **kwargs)

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py::test_dlpack_versioned_roundtrip[array]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py::test_dlpack_versioned_roundtrip[sliced_array]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py::test_dlpack_versioned_roundtrip[empty_array]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py::test_dlpack_versioned_roundtrip[tensor]
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py:310: DeprecationWarning: Exporting an unversioned DLPack capsule is deprecated, pass max_version=(1, 0) or higher.
    expected = np.from_dlpack(DLPackForwarder(obj, max_version=None))

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_dlpack.py::test_dlpack_not_supported
  /opt/conda/envs/arrow/lib/python3.14/site-packages/_pytest/python.py:167: DeprecationWarning: Exporting an unversioned DLPack capsule is deprecated, pass max_version=(1, 0) or higher.
    result = testfunction(**testargs)

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_pandas.py::TestConvertMetadata::test_column_index_names_datetime[None]
opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_pandas.py::TestConvertMetadata::test_column_index_names_datetime[Europe/Brussels]
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_pandas.py:126: Pandas4Warning: assert_index_equal will check the 'freq' attribute by default in a future version; these freqs None and <50 * Days> do not match. Pass check_freq=True or check_freq=False to silence this warning
    tm.assert_frame_equal(result, expected, check_dtype=check_dtype,

opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_pandas.py::test_does_not_mutate_timedelta_dtype
  /opt/conda/envs/arrow/lib/python3.14/site-packages/pyarrow/tests/test_pandas.py:5009: DeprecationWarning: The 'generic' unit for NumPy timedelta is deprecated, and will raise an error in the future. Please use a specific unit instead.
    df = pd.DataFrame({"a": [np.timedelta64()]})

There are some other warnings related to "Series.values returning an ndarray that drops timezone information for DatetimeTZDtype is deprecated ..." (and similar), but that actually requires a code change and not just a test update, so will open a separate issue for that (-> #51302).