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