correct attribute name under py3

gevent/monkey.py

@@ -163,15 +163,20 @@ def _patch_existing_locks(threading):
     # locked objects...at least if they use `with`. To be sure, we look at every object
     # Since we're supposed to be done very early in the process, there shouldn't be
     # too many.
+
+    # By definition there's only one thread running, so the various
+    # owner attributes were the old (native) thread id. Make it our
+    # current greenlet id so that when it wants to unlock and compare
+    # self.__owner with _get_ident(), they match.
     gc = __import__('gc')
     for o in gc.get_objects():
         if isinstance(o, rlock_type):
-            if o._RLock__owner is not None:
-                # By definition there's only one thread running,
-                # so this was the old (native) thread id. Make
-                # it our current greenlet id so that when it wants to unlock
-                # and compare self.__owner with _get_ident(), they match
-                o._RLock__owner = tid
+            if hasattr(o, '_owner'): # Py3
+                if o._owner is not None:
+                    o._owner = tid
+            else:
+                if o._RLock__owner is not None:
+                    o._RLock__owner = tid
         elif isinstance(o, _ModuleLock):
             if o.owner is not None:
                 o.owner = tid