[py3] Replaced __nonzero__ by __bool__

Of course, __nonzero__ alias has been kept for Python 2 compatibility.

django/dispatch/saferef.py

@@ -152,9 +152,10 @@ class BoundMethodWeakref(object):
     
     __repr__ = __str__
     
-    def __nonzero__( self ):
+    def __bool__( self ):
         """Whether we are still a valid reference"""
         return self() is not None
+    __nonzero__ = __bool__ # Python 2
 
     def __eq__(self, other):
         """Compare with another reference"""