Results: add __bool__ method.

Python 3 dropped __nonzero__ in preference to __bool__.  The code was
only working because they fall back to __len__ if __bool__ is not
implemented.  This commit adds a __bool__ implementation that simply
calls __nonzero__.

docs/history.rst

 Version History
 ===============
+- Next Release
+  - Implement ``Results.__bool__`` to be explicit about Python 3 support.
 - 1.10.3 2017-11-01
   - Remove the functionality from ``TornadoSession.validate`` and make it raise a ``DeprecationWarning``
   - Catch the ``KeyError`` raised when ``PoolManager.clean()`` is invoked for a pool that doesn't exist

queries/results.py

@@ -59,6 +59,9 @@ class Results(object):
     def __nonzero__(self):
         return bool(self.cursor.rowcount)
+    def __bool__(self):
+        return self.__nonzero__()
+
     def __repr__(self):
         return '<queries.%s rows=%s>' % (self.__class__.__name__, len(self))