Use Literal in a few more places (#3176)

stdlib/3/subprocess.pyi

@@ -7,9 +7,13 @@ from typing import (
     Generic, TypeVar, AnyStr,
     overload,
 )
-from typing_extensions import Literal
 from types import TracebackType
 
+if sys.version_info >= (3, 8):
+    from typing import Literal
+else:
+    from typing_extensions import Literal
+
 # We prefer to annotate inputs to methods (eg subprocess.check_call) with these
 # union types.
 # For outputs we use laborious literal based overloads to try to determine