Drop Python 2 support

[itamarst]

Accidental Python 2 incompatibility is wasting my time, and inability to use Python 3 features (even just in new modules) is somewhat frustrating.

Python 3 support has been out for 4 months, and there have been three releases with Python 2+3 support already. If Python 2 fixes are needed they can be done off a branch taken from the latest one of those.

Proposed plan for this particular iteration:

• Remove all Python 2 builders.
• Only allow Python 3 features in new modules (i.e. HTTP storage) for now, so that backports of critical bug fixes and security updates can happen for a few more months.