arrow

ARROW-17829: [Python] Avoid pandas groupby deprecation warning write_to_dataset (#14306)

This PR is looking at the deprecation warning from Pandas 1.5.0 connected to length 1 tuple used in the `groupby()` operation, see https://github.com/pandas-dev/pandas/issues/42795.

Currently we use `groupby` operation with a length 1 tuple in:
- `multisourcefs()` fixture in `test_dataset.py`:
https://github.com/apache/arrow/blob/466018084861f86a9171803c6d689e9c1c1efb5a/python/pyarrow/tests/test_dataset.py#L197
- `write_to_dataset()` in `pyarrow/parquet/core.py`:
https://github.com/apache/arrow/blob/466018084861f86a9171803c6d689e9c1c1efb5a/python/pyarrow/parquet/core.py#L3346-L3348

This PR fixes the the test and adds a check in `parquet/core.py` to avoid the warning.

Lead-authored-by: Alenka Frim <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>