Drop support for Python 2.6

[jaraco]

As pip has dropped support for Python 2.6, setuptools should do so as well.

[xavfernandez]

Erm, pip will drop 2.6 support in pip 10, i.e. in >=6 months.
pip will likely have a 9.1 release in between, with support for 2.6 ⭕️

cf pypa/pip#3955

[jaraco]

Oh my. Okay. I was confused. I'll shelve this change for later then.

[jaraco]

I've moved the commits to the drop-py26 branch, to be merged at an appropriate time.