Python 3.3 support dropped in a minor version

[dmtucker]

This is a backward-incompatible change ☹️

[dmtucker]

I see this was discussed a little in #165 (comment), but I would like to point out that pinning can also be done with the compatible release operator (e.g. ~=1.0 or ~=2.0). In other words, some dependents may have already pinned long ago.