refactor: delegate _attribute_is_optional to types._is_optional

Address review feedback on #295. Removes the duplicated dual-origin
check from _attribute_is_optional; the two functions now share the
single implementation in types._is_optional.

Minor semantic tightening: Union[A, B, None]-style fields (3+ args)
now return False from _attribute_is_optional, matching _is_optional's
exactly-two-args rule. Tracked separately in a follow-up issue.

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

Author: msto

fgpyo/util/inspect.py

@@ -24,7 +24,6 @@
 from typing import TypeAlias
 from typing import TypeGuard
 from typing import TypeVar
-from typing import Union
 
 import fgpyo.util.types as types
 
@@ -491,10 +490,7 @@ def attr_from(
 
 def _attribute_is_optional(attribute: FieldType) -> bool:
     """Returns True if the attribute is optional, False otherwise."""
-    origin = typing.get_origin(attribute.type)
-    return (origin is Union or origin is python_types.UnionType) and isinstance(
-        None, typing.get_args(attribute.type)
-    )
+    return types._is_optional(attribute.type)
 
 
 def _attribute_has_default(attribute: FieldType) -> bool: