Enable absolute imports, true division, & print()

These have no effect in Python 3.x, they are the default. Enabling them
in Python 2.x, enabling them in Python 2.x allows single source
compatiblity.

examples/base64_sem1.py

@@ -13,7 +13,7 @@
 # transform the parse tree in small steps in a bottom-up fashion. Compare
 # base64_sem2.py for an alternative approach using a single top-level action.
 
-from __future__ import print_function
+from __future__ import absolute_import, division, print_function
 
 import functools
 import sys