Revamp development workflow

[pradyunsg]

• Cleanup unnecessary files from the root of the repository
• Move to using nox
• Significantly update Travis CI configuration

[pradyunsg]

I'm gonna assume that everyone is OK to add nox (used for automation in https://github.com/pypa/packaging/, https://github.com/pypa/pip/ and more) and to revamp our CI to... well, actually work. All my follow up PRs will be based off this, just to make my own life easier. :)

[palnabarun]

Put in some comments.

Otherwise, LGTM!

[palnabarun]

Is there any reason for choosing 3.5?

[pradyunsg]

It works -- every other version seems to have some issue with the outdated-and-pinned dependencies.

[palnabarun]

Ah. Okay. Will keep a note to update this in future if needed.

[palnabarun]

This LGTM now!

[palnabarun]

@pradyunsg One nit though, can we keep the Dockerfile and docker-compose.yml for now and explicitly mark them as deprecated?

[palnabarun]

We can come back later to fix them.

[pradyunsg]

I'm going to go ahead and merge this in, since it unblocks our CI (when Travis isn't acting up), there's a green tick, and only a nit pointed out after.

re Docker: I prefer keeping things in master that are "known working" -- since it is at the back of a whole bunch of folks' minds (and a tracking issue exists) for working on the Docker setup, it definitely won't hurt to remove these outright now (when they're not working) and add a working variant later. If anyone wants to add back the not-working Dockerfile later, we can do it in another PR too (I won't block that in any way, even though I think that would be as not-useful as having a broken Dockerfile in master).