gh-117534: Add checking for input parameter in iso_to_ymd (#117543)

Moves the validation for invalid years in the C implementation of the `datetime` module into a common location between `fromisoformat` and `fromisocalendar`, which improves the error message and fixes a failed assertion when parsing invalid ISO 8601 years using one of the "ISO weeks" formats.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>