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

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

stdlib/3/http/server.pyi

@@ -1,7 +1,7 @@
 # Stubs for http.server (Python 3.4)
 
 import sys
-from typing import Any, BinaryIO, ClassVar, Dict, List, Mapping, Optional, Sequence, Tuple, Union
+from typing import Any, BinaryIO, Callable, ClassVar, Dict, List, Mapping, Optional, Sequence, Tuple, Union
 import socketserver
 import email.message
 
@@ -12,7 +12,7 @@ class HTTPServer(socketserver.TCPServer):
     server_name: str
     server_port: int
     def __init__(self, server_address: Tuple[str, int],
-                 RequestHandlerClass: type) -> None: ...
+                 RequestHandlerClass: Callable[..., BaseHTTPRequestHandler]) -> None: ...
 
 if sys.version_info >= (3, 7):
     class ThreadingHTTPServer(socketserver.ThreadingMixIn, HTTPServer):