Update to latest smart-open?

[zackmccord]

smart-open is now at version 7.0.4; pathy currently wants smart-open<7.0.0.

I thought I might update the dependency in requirements.txt and test it to see how it goes, but I see that getting the tests working in the first place is not altogether trivial.

Not sure why smart-open bumped its major version to 7, as the changes in 7.0.0 look fairly innocuous to my eye.