Use TypeAlias where possible for type aliases (#7630)

stdlib/socketserver.pyi

@@ -3,6 +3,7 @@ import types
 from _typeshed import Self
 from socket import socket as _socket
 from typing import Any, BinaryIO, Callable, ClassVar, Union
+from typing_extensions import TypeAlias
 
 if sys.platform == "win32":
     __all__ = [
@@ -36,8 +37,8 @@ else:
         "ThreadingUnixDatagramServer",
     ]
 
-_RequestType = Union[_socket, tuple[bytes, _socket]]
-_AddressType = Union[tuple[str, int], str]
+_RequestType: TypeAlias = Union[_socket, tuple[bytes, _socket]]
+_AddressType: TypeAlias = Union[tuple[str, int], str]
 
 # This can possibly be generic at some point:
 class BaseServer: