Add arrow.timezone() function for creating timezone objects

[rtsdque]

Summary

Adds arrow.timezone() as a public function for creating timezone objects directly from the arrow module, without needing to import from datetime, ZoneInfo, or dateutil.

Usage

import arrow

arrow.now(tz=arrow.timezone("US/Pacific"))
arrow.timezone("UTC")
arrow.timezone("local")

Changes

• arrow/api.py: Added timezone() function wrapping TzinfoParser.parse()
• arrow/__init__.py: Exported timezone from the public module API
• tests/test_api.py: Added 4 tests covering UTC, ZoneInfo, local, and invalid timezone strings

Testing

• All 1905 tests passing
• arrow/api.py at 100% coverage
• Overall coverage: 99.93% (above the 99% threshold)
• No new mypy errors introduced

Closes #912

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (b0f5c95).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1299   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2318    +3     
  Branches       358       358           
=========================================
+ Hits          2315      2318    +3     

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[rtsdque]

The linting issues have been fixed. The remaining failure (windows-latest pypy-3.11) is a network error during CI setup (IncompleteRead while downloading pyyaml), unrelated to this change. All 26 other checks pass including linting, codecov at 100% diff coverage, and tests across Python 3.8 through 3.13 on macOS, Ubuntu, and Windows.