Have you considered dropping support for CPython 2.6 (unsupported since 2013)?
Coverage.py has already dropped 3.2 (unsupported since 2016), but there were bigger gains for doing so (notably
u"" strings and Unicode support).
Still, here's some reasons for dropping 2.6: https://snarky.ca/stop-using-python-2-6/
Whilst we're here, how about dropping 3.3, which reaches end of life next week?