Add a PyICU timezone provider (#692) - #1755

Linked issue

• Contributes to Test with PyICU timezones [Python, guided by maintainer] #692

Description

Adds a PyICU timezone provider — a PYICU implementation of TZProvider, selectable via use_pyicu() and wired through TZP and the icalendar.timezone package. This is part 1 of #692 and is opened as a draft / work in progress.

Working so far (verified locally against the repo's VTIMEZONE fixtures — 37/38 create_timezone succeed):

• timezone lookup via icu.ICUtzinfo, UTC handling, and knows_timezone_id
• unknown ids return None, so the Windows→Olson and globally-unique-TZID fallbacks in TZP still work
• create_timezone parses the VTIMEZONE via icu.VTimeZone.createVTimeZone (DST transitions correct); a VTIMEZONE that ICU cannot parse raises ValueError instead of segfaulting the interpreter

Still to do (follow-up commits): parametrize conftest.py across the pyicu backend, add PyICU-specific tests, declare the optional PyICU test dependency (gated so it skips when ICU is absent), resolve uses_pyicu(), and lint/type-hint cleanup.

Questions I'd love input on, @niccokunzmann (I'll also add inline comments on the lines):

1. create_timezone uses icu.VTimeZone.createVTimeZone as you suggested. One fixture — issue_321_assert_dst_offset_is_not_false.ics (Europe/Berlin) — is rejected by ICU (empty id) where zoneinfo/pytz succeed. Prefer a fallback for those, or an xfail for pyicu?
2. uses_pyicu(): add it to the TZProvider ABC + TZP proxy for symmetry with uses_pytz/uses_zoneinfo, or keep providers reporting only pytz/zoneinfo?
3. PyICU is hard to install (especially in CI). How would you like the optional dependency and test-skipping handled?

Checklist

• I added a change log entry, following the instructions in all subsections under Change log requirements.
• I followed icalendar's Artificial intelligence policy and disclosed my Responsible AI use in my commit messages, if applicable.
• I added or updated tests, if applicable.
• I ran and ensured all tests pass locally by following Run tests.
• I added or edited documentation as necessary, both as docstrings to be rendered in the API documentation and narrative documentation, following the Style guide.

Additional information

Draft / WIP — tests, docs, and the conftest parametrization are coming in follow-up commits.

AI disclosure: I used Claude (Opus 4.8, via Claude Code) to explain the timezone-provider architecture and to verify the provider's behaviour by running it; the code is my own. This is also noted in the commit message and the news/692.feature entry.