Fix for nested()

IPython/utils/nested_context.py

@@ -4,6 +4,8 b' but it\'s removed in Python 3.2."""'
 # TODO : Remove this once we drop support for Python 2.6, and use
 # "with a, b:" instead.
 
+import sys
+
 from contextlib import contextmanager
 
 @contextmanager