Type socketserver's RequestHandlerClass as a callable. (#3422)

It's not uncommon to pass functions rather than actual types into the servers.

stdlib/2/SocketServer.pyi

@@ -1,14 +1,14 @@
 # NB: SocketServer.pyi and socketserver.pyi must remain consistent!
 # Stubs for socketserver
 
-from typing import Any, BinaryIO, Optional, Tuple, Type, Text, Union
+from typing import Any, BinaryIO, Callable, Optional, Tuple, Type, Text, Union
 from socket import SocketType
 import sys
 import types
 
 class BaseServer:
     address_family: int
-    RequestHandlerClass: type
+    RequestHandlerClass: Callable[..., BaseRequestHandler]
     server_address: Tuple[str, int]
     socket: SocketType
     allow_reuse_address: bool
@@ -16,7 +16,7 @@ class BaseServer:
     socket_type: int
     timeout: Optional[float]
     def __init__(self, server_address: Any,
-                 RequestHandlerClass: type) -> None: ...
+                 RequestHandlerClass: Callable[..., BaseRequestHandler]) -> None: ...
     def fileno(self) -> int: ...
     def handle_request(self) -> None: ...
     def serve_forever(self, poll_interval: float = ...) -> None: ...
@@ -44,23 +44,23 @@ class BaseServer:
 
 class TCPServer(BaseServer):
     def __init__(self, server_address: Tuple[str, int],
-                 RequestHandlerClass: type,
+                 RequestHandlerClass: Callable[..., BaseRequestHandler],
                  bind_and_activate: bool = ...) -> None: ...
 
 class UDPServer(BaseServer):
     def __init__(self, server_address: Tuple[str, int],
-                 RequestHandlerClass: type,
+                 RequestHandlerClass: Callable[..., BaseRequestHandler],
                  bind_and_activate: bool = ...) -> None: ...
 
 if sys.platform != 'win32':
     class UnixStreamServer(BaseServer):
         def __init__(self, server_address: Union[Text, bytes],
-                     RequestHandlerClass: type,
+                     RequestHandlerClass: Callable[..., BaseRequestHandler],
                      bind_and_activate: bool = ...) -> None: ...
 
     class UnixDatagramServer(BaseServer):
         def __init__(self, server_address: Union[Text, bytes],
-                     RequestHandlerClass: type,
+                     RequestHandlerClass: Callable[..., BaseRequestHandler],
                      bind_and_activate: bool = ...) -> None: ...
 
 class ForkingMixIn: ...