Replace __nonzero__ with __bool__ for Python 3.

On Python 2 alias __bool__ back to __nonzero__ and delete __bool__.
This allows SageObject._test_not_implemented_methods to pass on Python 2
and 3 for classes that implement this abstract interface.

Author: embray

src/sage/categories/additive_magmas.py

@@ -8,6 +8,8 @@
 #                  http://www.gnu.org/licenses/
 #******************************************************************************
 
+import six
+
 from sage.misc.lazy_import import LazyImport
 from sage.misc.abstract_method import abstract_method
 from sage.misc.cachefunc import cached_method
@@ -766,7 +768,7 @@ class ElementMethods:
             #     return self == self.parent().zero()
 
             @abstract_method
-            def __nonzero__(self):
+            def __bool__(self):
                 """
                 Return whether ``self`` is not zero.
 
@@ -785,6 +787,10 @@ def __nonzero__(self):
                     True
                  """
 
+            if six.PY2:
+                __nonzero__ = __bool__
+                del __bool__
+
             def _test_nonzero_equal(self, **options):
                 r"""
                 Test that ``.__bool__()`` behave consistently