feat: replace python-dateutil with stdlib datetime.fromisoformat

[splch]

Summary

Drop the python-dateutil dependency from both the generator and all generated client packages. Date/datetime parsing now uses Python's built-in datetime.fromisoformat() instead of dateutil.parser.isoparse().

This removes one runtime dependency from every generated client, reducing install size and eliminating a dependency that is unmaintained upstream and being deprecated by Fedora 45 (no releases since March 2024).

Changes

Generator source (openapi_python_client/parser/properties/):

• datetime.py: Validate defaults with datetime.datetime.fromisoformat() instead of isoparse(). Normalize Z to +00:00 at generation time so the emitted Python code is clean.
• date.py: Validate defaults with datetime.date.fromisoformat() instead of isoparse().date().
• Both: Remove "from dateutil.parser import isoparse" from the import set returned by get_imports().

Jinja templates (openapi_python_client/templates/property_templates/):

• datetime_property.py.jinja: isoparse(x) -> datetime.datetime.fromisoformat(x.replace("Z", "+00:00"))
• date_property.py.jinja: isoparse(x).date() -> datetime.date.fromisoformat(x)

Dependency removal (all 4 metadata formats):

• pyproject_uv.toml.jinja, pyproject_poetry.toml.jinja, pyproject_pdm.toml.jinja, setup.py.jinja: Remove python-dateutil from generated dependencies.
• pyproject.toml (generator): Remove python-dateutil from runtime deps and types-python-dateutil from dev deps.
• integration-tests/pyproject.toml: Remove both as well.

Tests & golden records: All regenerated. Unit tests pass (283 passed, 4 skipped).

Python 3.10 compatibility

datetime.fromisoformat() gained full ISO 8601 support (including the Z suffix) in Python 3.11. On Python 3.10, the Z suffix raises a ValueError:

# Python 3.10
>>> datetime.datetime.fromisoformat("2024-01-15T10:30:00Z")
ValueError: Invalid isoformat string: '2024-01-15T10:30:00Z'

>>> datetime.datetime.fromisoformat("2024-01-15T10:30:00+00:00")
datetime.datetime(2024, 1, 15, 10, 30, tzinfo=datetime.timezone.utc)  # works

# Python 3.11+
>>> datetime.datetime.fromisoformat("2024-01-15T10:30:00Z")
datetime.datetime(2024, 1, 15, 10, 30, tzinfo=datetime.timezone.utc)  # works natively

The generated datetime parsing code uses .replace("Z", "+00:00") to normalize Z to an explicit UTC offset before calling fromisoformat(), which works on both 3.10 and 3.11+. This is a no-op on strings without Z. Date parsing does not need this since date strings have no timezone component.

Default values in OpenAPI specs are normalized at generation time (Z -> +00:00), so the emitted default expressions are clean datetime.datetime.fromisoformat("...") calls.