style(forms): use X | Y unions in isinstance (UP038)

ruff v0.6.9 pinned in pre-commit-config still enforces UP038,
which my local newer ruff treats as deprecated/off-by-default,
so the rule slipped past the local sweep but tripped CI's
pre-commit job.

Python 3.10+ supports 'isinstance(x, A | B)' natively, and
'requires-python = >=3.10' so this is the canonical form
anyway.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mpasternak

.claude/scheduled_tasks.lock

@@ -0,0 +1 @@
+{"sessionId":"610a2b24-c79a-48e3-b1d3-30d9d64822ea","pid":23539,"procStart":"156828","acquiredAt":1778477032624}

src/formdefaults/forms.py

@@ -15,9 +15,9 @@ def _serialize(value):
 
     Covers the common Django field types whose `Form.initial` is JSON-able
     when snapshotted. For unsupported types, falls back to `str(value)`."""
-    if value is None or isinstance(value, (bool, int, float, str, list, dict)):
+    if value is None or isinstance(value, bool | int | float | str | list | dict):
         return value
-    if isinstance(value, (datetime.date, datetime.datetime, datetime.time)):
+    if isinstance(value, datetime.date | datetime.datetime | datetime.time):
         return value.isoformat()
     if hasattr(value, "pk"):  # ModelChoiceField etc.
         return value.pk