Upgrade to Thumbor 7 (and Python 3)

[scorphus]

Proposal

This PR includes several changes to the codebase not only to make it compatible with Thumbor 7 and Python 3 but also to put the project back on track.

There's some stuff left to do – I just ran out of Ben & Jerry's – but I'd love to collect early reviews, so I'm submitting this as draft.

TODO

Here's a list of things to do (there might be more) starting from the easiest:

• Check code with linters as part of the workflow
• Add thumbor as Git submodule and use its integration tests base structure instead of repeating content inside ./integration_tests/
• Write unit tests (there is none)

These could also be converted into issues and tackled by any avid bootcamper.

[neilsh]

I know this hasn't been worked on recently @scorphus , but were there any known issues/incompatibilities? I'm currently upgrading an internal codebase that uses opencv-engine from Thumbor 6->7, and was wondering if this would be suitable for use. If there's any known issues I can take a crack at resolving them.