fix UnionType.__or__ and add UnionType.__getitem__ (#14687)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stdlib/@tests/test_cases/check_types.py

@@ -3,9 +3,11 @@ from __future__ import annotations
 import sys
 import types
 from collections import UserDict
-from typing import Union
+from typing import Any, Literal, TypeVar, Union
 from typing_extensions import assert_type
 
+_T = TypeVar("_T")
+
 # test `types.SimpleNamespace`
 
 # Valid:
@@ -58,3 +60,13 @@ class DCAtest:
     @foo.deleter
     def foo(self) -> None:
         self._value = None
+
+
+if sys.version_info > (3, 10):
+    union_type = int | list[_T]
+
+    # ideally this would be `_SpecialForm` (Union)
+    assert_type(union_type | Literal[1], types.UnionType | Any)
+    # Both mypy and pyright special-case this operation,
+    # but in different ways, so we just check that no error is emitted:
+    _ = union_type[int]