Fix parsing of the Y year token in from_format()

[vera-rubin]

We get KeyError: 'Y' when parsing 2024-02-29 with Y-MM-DD, even though the documented year tokens include Y. This should return 2024-02-29T00:00:00+00:00.

import pendulum
print(pendulum.from_format("2024-02-29", "Y-MM-DD").isoformat())

The change adds integer conversion for Y alongside YYYY and YY. We added a regression case and ran the full test suite with the Release extension and with PENDULUM_EXTENSIONS=0.

Tested: Pendulum 3.3.0, master at e06dacedc4cdea90fa1286b3965d180ab94cf251.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code. The existing documentation already lists Y.

— ChipLab

[Wkhanex]

I checked e77a6d7 with Python 3.12 in pure-Python mode. In addition to the 62 passing tests/datetime/test_from_format.py tests, I compared from_format(..., 'Y-MM-DD') against datetime for January 2, February 28 and December 31 of every year 1–9999 (29,997 comparisons). Those passed, including unpadded years such as 68 and 69 remaining literal years rather than receiving the YY pivot. I also checked that the existing YY pivot stays at 2068/1969 and that year 0, year 10000 and non-leap February 29 are rejected. This gives the one-entry token-map fix some coverage beyond the single leap-year example. I have not run the complete suite or the native extension backend.

Review and local validation prepared with OpenAI Codex assistance.