Fix issue when typing_extensions is not installed

Author: wyfo

apischema/typing.py

@@ -70,7 +70,7 @@ def get_args(tp):  # type: ignore
     try:
         from typing_extensions import Literal, TypedDict, Protocol  # noqa F401
     except ImportError:
-        Literal, TypedDict = _FakeType, _FakeType
+        pass
 
 
 _T = TypeVar("_T")