Add BaseRequestHandler instance attributes

From the BaseRequestHandler documentation:

    handle()

    This function must do all the work required to service a request.
    The default implementation does nothing. Several instance attributes
    are available to it; the request is available as self.request; the
    client address as self.client_address; and the server instance as
    self.server, in case it needs access to per-server information.

    The type of self.request is different for datagram or stream
    services. For stream services, self.request is a socket object;
    for datagram services, self.request is a pair of string and
    socket.

I also added a union that expresses the fact that client_address can be
a string in case of using Unix domain sockets.

Author: Jakub Stasiak

stdlib/3/socketserver.pyi

@@ -69,6 +69,9 @@ class ThreadingTCPServer(ThreadingMixIn, TCPServer): ...
 class ThreadingUDPServer(ThreadingMixIn, UDPServer): ...
 
 class BaseRequestHandler:
+    request = ...  # type: Union[SocketType, Tuple[bytes, SocketType]]
+    client_address = ...  # type: Union[Tuple[str, int], str]
+    server = ...  # type: BaseServer
     def setup(self) -> None: ...
     def handle(self) -> None: ...
     def finish(self) -> None: ...