Fix TypeError crash on failed imports under py3.3.

Starting with Python 3.3, NamespacePath passed to importlib hooks
seem to have lost the ability to be accessed by index.

We wrap the index access in a try..except and wrap the path in a
list if it happens.

Fixes #383.

_pytest/assertion/rewrite.py

@@ -57,7 +57,12 @@ class AssertionRewritingHook(object):
         lastname = names[-1]
         pth = None
         if path is not None and len(path) == 1:
-            pth = path[0]
+            try:
+                pth = path[0]
+            except TypeError:
+                # Starting with Python 3.3, `path` started being unsubscriptable, we have to wrap it
+                # in a list.
+                pth = list(path)[0]
         if pth is None:
             try:
                 fd, fn, desc = imp.find_module(lastname, path)