setup.py uses sys.version_info.major which was introduced at Python2.7 (thanks to Jeremy Lewi for reporting).
Note that I haven't backported this; just checked it into trunk.
I've reviewed and committed this. Thanks for the bug fix. I believe the bug was introduced in AVRO-833; my apologies for not spotting it in review.
Patch to make setup.py compatible with version < 2.7