Fix Python 2.6 incompatibility introduced in 4e77027.

sphinx/pycode/pgen2/pgen.py

@@ -4,7 +4,10 @@
 from __future__ import print_function
 
 from six import iteritems
-from collections import OrderedDict
+try:
+    from collections import OrderedDict
+except ImportError: # Fallback for Python 2.6
+    OrderedDict = dict
 
 # Pgen imports