Fix a broken assert.

Python gave a warning when starting hydra:

SyntaxWarning: assertion is always true, perhaps remove parentheses?

The previous syntax was a 2-element tuple, always true.

pygal/util/__init__.py

@@ -143,7 +143,7 @@ class iterable_test(dict):
         """ignore_classes must include basestring, because if a string
         is iterable, so is a single character, and the routine runs
         into an infinite recursion"""
-        assert (basestring in ignore_classes,
+        assert basestring in ignore_classes, (
                 'basestring must be in ignore_classes')
         self.ignore_classes = ignore_classes