Drop Python 2 support

[mdickinson]

We no longer need to support Python 2; related code can be dropped.

[aaronayres35]

What is the status of this issue? I'm not sure what else needs to be done to say we have officially dropped python 2 support

[mdickinson]

It's looking good; getting rid of six was the single biggest thing (done in #230).

Taking a quick look, I spotted a couple of leftovers (but maybe there's still a sweet_pickle PR pending?)

• there's still one from __future__ import print_function in apptools/sweet_pickle/tests
• there's an if sys.version_info[0] >= 3: in the apptools/sweet_pickle/versioned_unpickler.py

[mdickinson]

but maybe there's still a sweet_pickle PR pending?

Ah yes: #199. I'll close this, then.