Use typing_extensions.Self instead of _typeshed.Self (#9702)

stubs/pyRFC3339/pyrfc3339/utils.pyi

@@ -1,13 +1,13 @@
-from _typeshed import Self
 from datetime import datetime, timedelta, tzinfo
 from typing import Any
+from typing_extensions import Self
 
 class FixedOffset(tzinfo):
     def __init__(self, hours: float, minutes: float) -> None: ...
     def dst(self, dt: datetime | None) -> timedelta: ...
     def utcoffset(self, dt: datetime | None) -> timedelta: ...
     def tzname(self, dt: datetime | None) -> str: ...
-    def __deepcopy__(self: Self, memo: dict[int, Any]) -> Self: ...
+    def __deepcopy__(self, memo: dict[int, Any]) -> Self: ...
 
 def timedelta_seconds(td: timedelta) -> int: ...
 def timezone(utcoffset: float) -> str: ...