Summary

ua is an ISO 3166 country code (Ukraine), not the ISO 639 language code for Ukrainian, which is uk. This mismatch means:

• pendulum.now().diff_for_humans(locale='uk') raises ValueError: Locale [uk] does not exist., even though uk is what locale.getlocale() and Accept-Language HTTP headers actually report (e.g. uk_UA, uk-UA).
• Only the non-standard locale='ua' code worked.

Fix

• Renamed src/pendulum/locales/ua to src/pendulum/locales/uk (module docstrings/imports updated accordingly).
• Added an alias table to Locale.load() in src/pendulum/locales/locale.py so the old ua code still resolves to the same Ukrainian translations, preserving backwards compatibility for existing callers.

Test plan

• Added tests/localization/test_uk.py:
  • test_diff_for_humans exercises diff_for_humans(locale='uk').
  • test_ua_is_an_alias_for_uk asserts Locale.load('ua') and Locale.load('uk') produce identical translations, and that diff_for_humans(locale='ua') and diff_for_humans(locale='uk') match.
• Verified the new test fails on master (ModuleNotFoundError: No module named 'pendulum.locales.uk.locale') and passes after the fix.
• Ran tests/localization/ locally; failures there are pre-existing and unrelated (they require the compiled test extra for pendulum.travel_to, which couldn't be built in this environment due to an unrelated Rust/MSVC linker issue).

Fixes #955