Py3

Does what it says on the tin.
I confess it's difficult. Python dependencies are a fucking nightmare, especially transitive ones as in the case of a "library" like annotator-store.

Even if we don't pin explicitly as we do here (which I would agree is probably not the right thing to in a library) we should at least do something like semver pinning, i.e. `>=1.3.0,<1.4.0`.

(Although this still doesn't work, as `1.3.0 <= 1.4.0rc1 < 1.4.0`, so you actually have to do something disgusting like `>=1.3.0,<1.3.999`.)