Fix ISO 8601 week dates with years < 1000 - #1077

Fixes #1076.

Summary

pendulum.parse() rejected every ISO 8601 week date whose year is below 1000 — 0999-W34-6, 0416-W34-6, 0416W346, 0416-W34, 0004-W10-5 — with a generic ParserError, while datetime.fromisoformat accepts all of them and the same years parse fine in calendar form (0416-08-21). The boundary was exact: 1000-W34-6 worked, 0999-W34-6 did not.

Root cause

src/pendulum/parsing/iso8601.py, _get_iso_8601_week:

fmt = "%Y-%j"
string = f"{year}-{ordinal}"
dt = datetime.datetime.strptime(string, fmt)

The year is interpolated without zero-padding, so year 416 produces "416-240"; strptime's %Y matches exactly four digits, and the resulting ValueError is masked into the generic ParserError("Unable to parse string [...]") by _parse.

Fix

Pad the year to four digits: f"{year:04d}-{ordinal}".

Verification

• 5 regression cases added to the parametrized week-date test in tests/parsing/test_parse_iso8601.py (0999-W34-6, 0416-W34-6, 0004-W10-5, 0416W346, 0416-W34).
• Full tests/parsing suite: 88 passed (83 baseline + 5 new).
• Week-53 validation still intact: 0999-W53-7 correctly raises (999 is not a long year).
• tests/datetime/test_from_format.py failure set unchanged (the 49 pre-existing failures are the time-machine-gated travel tests, unavailable in this environment).
• Python 3.13, Linux.