Use _typeshed.Self with __enter__ (#5717)

Co-authored-by: Akuli <akuviljanen17@gmail.com>

stdlib/socketserver.pyi

@@ -1,5 +1,6 @@
 import sys
 import types
+from _typeshed import Self
 from socket import socket as _socket
 from typing import Any, BinaryIO, Callable, ClassVar, Optional, Set, Tuple, Type, TypeVar, Union
 
@@ -30,7 +31,7 @@ class BaseServer:
     def server_activate(self) -> None: ...
     def server_bind(self) -> None: ...
     def verify_request(self, request: _RequestType, client_address: _AddressType) -> bool: ...
-    def __enter__(self: _T) -> _T: ...
+    def __enter__(self: Self) -> Self: ...
     def __exit__(
         self, exc_type: Optional[Type[BaseException]], exc_val: Optional[BaseException], exc_tb: Optional[types.TracebackType]
     ) -> None: ...