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MINOR: [Python][CI] Fix harmless user warning in test_list_of_datetime_time_roundtrip (#36175)

### Rationale for this change

With recent pandas, this usage of `to_datetime` gives a warning about each element getting parsed individually, and that you can avoid the warning by passing a format:

```
>>> pd.to_datetime(['09:00', '09:30', '10:00', '10:30', '11:00', '11:30', '12:00'])
UserWarning: Could not infer format, so each element will be parsed individually, falling back to `dateutil`. To ensure parsing is consistent and as-expected, please specify a format.
```

This doesn't change any behaviour in the test, but just ensures running our tests don't print this warning.

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>