keepalive: switch from thread to threading module

The thread module in py3 is renamed to _thread, but we can use
 the high level threading module instead.

mercurial/keepalive.py

@@ -113,7 +113,7 b' import errno'
 import hashlib
 import socket
 import sys
-import thread
+import threading
 
 from . import (
     util,
@@ -135,7 +135,7 b' class ConnectionManager(object):'
       * keep track of all existing
       """
     def __init__(self):
-        self._lock = thread.allocate_lock()
+        self._lock = threading.Lock()
         self._hostmap = {} # map hosts to a list of connections
         self._connmap = {} # map connections to host
         self._readymap = {} # map connection to ready state