Python 3 compat.

Try to keep our Python sources compatible with both Python 2.7 and 3.

Check with 'pylint --py3k' and 'python -3'.

meta/isa/__init__.py

@@ -5,7 +5,7 @@ Cretonne target ISA definitions
 The :py:mod:`isa` package contains sub-packages for each target instruction set
 architecture supported by Cretonne.
 """
-
+from __future__ import absolute_import
 from . import riscv