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/gen_instr.py

@@ -1,7 +1,7 @@
 """
 Generate sources with instruction info.
 """
-
+from __future__ import absolute_import
 import srcgen
 import constant_hash
 from unique_table import UniqueTable, UniqueSeqTable