Replace pytz with standard library's zoneinfo

[markcampanelli]

This is a demonstration of how we could remove the direct dependency on pytz and adopt the IANA-compliant standard-library solution zoneinfo, available since Python 3.9. This also leads to a single-source-of-truth simplification of the timezone tracking in pvlib.location.Location. Note that pandas apparently still uses pytz extensively, but I did not find this to conflict.

TODO: [ ] Update the extensive timezone tutorial in timetimezones.rst.

• [ ] Closes #2342
• [ ] I am familiar with the contributing guidelines
• [ ] Tests added
• [ ] Updates entries in docs/sphinx/source/reference for API changes.
• [ ] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [ ] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [ ] Pull request is nearly complete and ready for detailed review.
• [ ] Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

[cwhanse]

@markcampanelli I'm marking this change for v0.13.0. I think the pytz is likely in many user workflows, and dropping it entirely should have sufficient deprecation. Let's deprecate pytz in v0.12.0 if not sooner.