Restore hashing for named timezones

[emme1t]

Fixes #1008.

Adding Timezone.__eq__ in 3.1.0 made named timezone objects unhashable. This breaks calls such as hash(pendulum.Timezone("UTC")) and using a timezone as a dictionary key or set element.

Implement __hash__ using the timezone key, matching the existing equality rule. The regression tests construct equal objects with Timezone.no_cache() and verify equal hashes, dictionary lookup, and set deduplication. A separate test checks dictionary lookup for two distinct timezone names.

Validation

Tested locally on Windows with Python 3.13.13:

• All four new regression cases fail on the original implementation.
• Timezone suite: 638 passed, 5 skipped.
• Complete pure-Python suite (PENDULUM_EXTENSIONS=0): 1,840 passed, 5 skipped, 2 failed. A clean checkout of the original commit produces the same two failures, with 1,836 passing tests. The failing tests are tests/date/test_diff.py::test_diff_for_humans_now_and_future_month and tests/test_parsing.py::test_parse.
• The project's pinned Ruff 0.14.11 lint and format checks pass for both changed files.
• The lockfile's Mypy 1.13.0 passes for all 223 source/test files.
• git diff --check passes.

The native extension build could not run on this machine because the MSVC link.exe linker is unavailable. The Python source tests use the same pure-Python mode exercised by the repository's CI.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code (existing behavior is restored; no user-facing option changes).

This patch, regression tests, and PR description were prepared with OpenAI Codex assistance. The reported reproductions and tests were executed locally.

[ashb]

This is slightly incorrect, as it means that "UTC" and Timezone("UTC") would hash to the same value

[emme1t]

Thanks for pointing this out. Updated in b623446 to hash (Timezone, self.key), giving named timezones a type-qualified hash input. Using the base Timezone class also preserves equal hashes for subclasses that inherit the existing equality behavior.

Added regression coverage for equal base/subclass instances as dictionary keys and set members across UTC, Europe/Paris, and America/New_York.

Local validation on Windows / CPython 3.13 in pure-Python mode: 641 timezone tests passed, 5 skipped. The full suite has 1,843 passed, 5 skipped, and the same two failures reproduced on the original upstream checkout (test_diff_for_humans_now_and_future_month and test_parse). Mypy 1.13.0 passes across 223 files. The Rust extension was not validated.

Ruff 0.14.11 lint and format checks pass as well. This patch and validation were prepared with Codex assistance.