python-cryptography is a dependency for python-acme (and in turn letsencrypt). I can get python-idna building on EL6 (python 2.6), but it fails on the unit tests. I'm not too sure whether this is a failing in the module itself or in the tests.
reported against wrong component, I think this is supposed to be against python-cryptography
bug 1288998 comment 2
Itamar, the bug is not incorrectly assigned, but badly worded. letsencrypt in EPEL6 would require python-acme, which would require python-cryptography, which would require python-idna. Working on this, I was able to reproduce this issue. The tests fail with the latest master as well in Python 2.6. I'm not sure if Python 2.6 is even supposed to work with the project and have opened an issue upstream. Getting the tests fixed and a confirmation would be a prerequisite to epel6 submission. https://github.com/kjd/idna/issues/20
Not really an issue anymore as we moved on to CentOS 7. Probably not worth the time to get this working on EL6/CentOS 6