Fix test_httplib when built without threads

Lib/test/ssl_servers.py

@@ -3,13 +3,13 @@ import sys
 import ssl
 import pprint
 import socket
-import threading
 import urllib.parse
 # Rename HTTPServer to _HTTPServer so as to avoid confusion with HTTPSServer.
 from http.server import (HTTPServer as _HTTPServer,
     SimpleHTTPRequestHandler, BaseHTTPRequestHandler)
 
 from test import support
+threading = support.import_module("threading")
 
 here = os.path.dirname(__file__)