Update typing_extensions imports in third-party stubs (#11245)

stubs/python-dateutil/dateutil/easter.pyi

@@ -1,5 +1,5 @@
 from datetime import date
-from typing_extensions import Literal
+from typing import Literal
 
 EASTER_JULIAN: Literal[1]
 EASTER_ORTHODOX: Literal[2]

stubs/python-dateutil/dateutil/tz/tz.pyi

@@ -1,7 +1,6 @@
 import datetime
 from _typeshed import Incomplete
-from typing import ClassVar, Protocol, TypeVar
-from typing_extensions import Literal
+from typing import ClassVar, Literal, Protocol, TypeVar
 
 from ..relativedelta import relativedelta
 from ._common import _tzinfo as _tzinfo, enfold as enfold, tzname_in_python2 as tzname_in_python2, tzrangebase as tzrangebase