Fix contextlib imports for python3 as per @takluyver's review.

IPython/core/interactiveshell.py

@@ -28,8 +28,13 b' import runpy'
 import sys
 import tempfile
 import types
-import urllib
-from io import open as io_open
+
+# We need to use nested to support python 2.6, once we move to >=2.7, we can
+# use the with keyword's new builtin support for nested managers
+try:
+    from contextlib import nested
+except:
+    from IPython.utils.nested_context import nested
 
 from IPython.config.configurable import SingletonConfigurable
 from IPython.core import debugger, oinspect

IPython/frontend/terminal/embed.py

@@ -25,9 +25,15 b' Notes'
 from __future__ import with_statement
 
 import sys
-from contextlib import nested
 import warnings
 
+# We need to use nested to support python 2.6, once we move to >=2.7, we can
+# use the with keyword's new builtin support for nested managers
+try:
+    from contextlib import nested
+except:
+    from IPython.utils.nested_context import nested
+
 from IPython.core import ultratb
 from IPython.core.magic import Magics, magics_class, line_magic
 from IPython.frontend.terminal.interactiveshell import TerminalInteractiveShell

IPython/frontend/terminal/interactiveshell.py

@@ -20,7 +20,12 b' import re'
 import sys
 import textwrap
 
-from contextlib import nested
+# We need to use nested to support python 2.6, once we move to >=2.7, we can
+# use the with keyword's new builtin support for nested managers
+try:
+    from contextlib import nested
+except:
+    from IPython.utils.nested_context import nested
 
 from IPython.core.error import TryNext, UsageError
 from IPython.core.usage import interactive_usage, default_banner