Use zoneinfo for timezone handling instead of pytz

[saranti]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• [ ] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [ ] 📚 Updated documentation for changed code.
• [x] ⏩ Code is up-to-date with the master branch.

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Description of Changes

Closes #1175. Remove all pytz code and replace it with the equivalent from zoneinfo and use a backport for python versions <3.9.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.91%. Comparing base (87a1a77) to head (d3fe586). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1179 +/- ## =========================================== - Coverage 100.00% 99.91% -0.09% =========================================== Files 10 10 Lines 2326 2324 -2 Branches 400 400 =========================================== - Hits 2326 2322 -4 - Misses 0 1 +1 - Partials 0 1 +1 ```

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