GH-15047: [Python]: switch from pytz to zoneinfo by default for string to tzinfo conversion

[jorisvandenbossche]

Rationale for this change

zoneinfo is available starting with Python 3.9, so we can now assume that it is available, and so we can switch from by default returning pytz timezones to return zoneinfo timezones (or datetime.timezone for fixed offsets).

Only keeping pytz as fallback for strings that are not supported by zoneinfo but were supported by pytz. Later, we should maybe deprecate that fallback.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

This PR includes breaking changes to public APIs.

It is a different object that we return (different class), which a different API, so for people relying on that, this is a breaking change.

• GitHub Issue: [Python] Drop pytz for timezone support (default to use datetime.timezone / zoneinfo) #15047

[github-actions]

⚠️ GitHub issue #15047 has been automatically assigned in GitHub to PR creator.

[jorisvandenbossche]

@github-actions crossbow submit test-pandas

[github-actions]

Revision: dc184fe

Submitted crossbow builds: ursacomputing/crossbow @ actions-cdf269df89

Task	Status
test-conda-python-3.10-pandas-1.3.4-numpy-1.21.2
test-conda-python-3.11-pandas-latest-numpy-latest
test-conda-python-3.12-pandas-latest-numpy-1.26
test-conda-python-3.12-pandas-latest-numpy-latest
test-conda-python-3.13-pandas-nightly-numpy-nightly
test-conda-python-3.13-pandas-upstream_devel-numpy-nightly