[Python] Support pandas future default string dtype

[jorisvandenbossche]

With pandas' PDEP-14 proposal, pandas is planning to introduce a default string dtype in pandas 3.0 (instead of the current object dtype).

This will become the default in pandas 3.0, and can be enabled with an option in the upcoming pandas 2.3 (pd.options.future.infer_string = True). To prepare for that, we should start using that string dtype in to_pandas() conversions when that option is enabled.

[raulcd]

@jorisvandenbossche should I move this to 19.0.0?