Abstract `ast.AST` classes do not have `_field_types`

[hunterhogan]

Only concrete ast.AST classes have _field_types, but because it is defined on ast.AST, IDEs, for example, incorrectly autocomplete it on abstract classes.

typeshed/stdlib/ast.pyi

Lines 32 to 38 in 2f198bc

	class AST:
	if sys.version_info >= (3, 10):
	__match_args__ = ()
	_attributes: ClassVar[tuple[str, ...]]
	_fields: ClassVar[tuple[str, ...]]
	if sys.version_info >= (3, 13):
	_field_types: ClassVar[dict[str, Any]]