Issue #23358: Add missing BaseServer entry to socketserver.__all__.

Patch by Martin Panter.
diff --git a/Lib/socketserver.py b/Lib/socketserver.py
index 7339911..5cb89be 100644
--- a/Lib/socketserver.py
+++ b/Lib/socketserver.py
@@ -138,10 +138,10 @@
 except ImportError:
     import dummy_threading as threading
 
-__all__ = ["TCPServer","UDPServer","ForkingUDPServer","ForkingTCPServer",
-           "ThreadingUDPServer","ThreadingTCPServer","BaseRequestHandler",
-           "StreamRequestHandler","DatagramRequestHandler",
-           "ThreadingMixIn", "ForkingMixIn"]
+__all__ = ["BaseServer", "TCPServer", "UDPServer", "ForkingUDPServer",
+           "ForkingTCPServer", "ThreadingUDPServer", "ThreadingTCPServer",
+           "BaseRequestHandler", "StreamRequestHandler",
+           "DatagramRequestHandler", "ThreadingMixIn", "ForkingMixIn"]
 if hasattr(socket, "AF_UNIX"):
     __all__.extend(["UnixStreamServer","UnixDatagramServer",
                     "ThreadingUnixStreamServer",