arrow

ARROW-15580: [Python] Make pytz an actual optional dependency of PyArrow

This PR tries to change `pytz` module to be an optional dependency.

The tests that install/use `pytz` had to be changed.

I ran all `pyarrow` tests locally with and without `pytz` installed. We could update the build without Pandas to not include `pytz` either so the code could be checked on the CI also.

- [x] Change `pytz` to an optional dependency
- [x] Change the use of `PyObject_HasAttrString` to use `PyObject_IsInstance`

Closes #12522 from AlenkaF/ARROW-15580

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