Thumbor 7 needs Python 3

[Soviut]

In their release notes for Thumbor 7 they mention breaking changes including the move to Python 3.

https://github.com/thumbor/thumbor/releases

This release is still in alpha so it isn't critical to support it, but it would be nice to be ready when it drops.

[ghchinoy]

@Soviut Thank you for the heads up! I'll make a point to keep an eye on the thumbor 7 release progress.

Deployment with 7 alpha on branch thumbor7