Support get_origin(Union[...]) in python 3.14

In Python 3.14, `Union` is no longer a `typingGenericAlias`, so
`get_origin(int | str)` didn't return `Union`. This changes that by
explicitly checking for `is_union_type`. This adds a test for
`get_origin(Union[...])`.

Author: leon-sony

test_typing_inspect.py

@@ -355,6 +355,7 @@ def test_origin(self):
         if GENERIC_TUPLE_PARAMETRIZABLE:
             tp = List[Tuple[T, T]][int]
             self.assertEqual(get_origin(tp), list if NEW_TYPING else List)
+        self.assertIs(get_origin(Union[str, int]), typing.Union)
 
     def test_parameters(self):
         T = TypeVar('T')

typing_inspect.py

@@ -359,7 +359,13 @@ def get_origin(tp):
         get_origin(List[Tuple[T, T]][int]) == list  # List prior to Python 3.7
     """
     if NEW_TYPING:
-        if isinstance(tp, typingGenericAlias):
+        if (
+            isinstance(tp, typingGenericAlias)
+            # In Python 3.14, Union[...] is not an instance of
+            # typingGenericAlias, but it does have an `__origin__`, so check for
+            # Unions explicitly.
+            or is_union_type(tp)
+        ):
             return tp.__origin__ if tp.__origin__ is not ClassVar else None
         if tp is Generic:
             return Generic