Fix or remove Python 2.6 support

[gtback]

As pointed out by @mbekavac, the clients.py module hasn't worked in Python 2.6 in a long time, but we haven't noticed since there aren't really any tests for that module. The rest of libtaxii seems to work on Python 2.6 (at least according to CI results)

We should add tests for the clients module, or drop explicit support for Python 2.6.