Use Poetry for dependency management

[bpepple]

Was looking at hacking a bit on pylistenbrainz, and figured I'd make it easier for myself by using Poetry. Not sure if you have any interest in using it, but in case you do here's a pull request.

Some of the changes include:

• Move tests into parent directory, so they are include in source tarball, and not included in the wheel.
• Drop unsupported versions of Python (3.5, 3.6).
• Make Sphinx installation optional.

The Travis config will need to be modified to use Poetry (and probably have the python versions tested updated).

Any questions, don't hesitate to ask. Thx.

[georgkrause]

@bpepple I just migrated the project from setup.cfg to pyproject.toml, which should lower the distance for your changes a bit. It also contains a solution for the dynamic version. The musicbrainz team does not have a decision regarding poetry yet, but in case you are up to it a rebase would be appreciated I guess :)

Disclaimer: I am not part of the team, just contributed some packaging today

[georgkrause]

@bpepple Are you interested in proceeding here? I could pick up what you started, however I don't want to steal anything :) Let me know!

[bpepple]

@bpepple Are you interested in proceeding here? I could pick up what you started, however I don't want to steal anything :) Let me know!

Nah, I'm fine with you taking over.

[georgkrause]

I ported the extended URLs and the move of the tests in #29

After discussions with @phw and @mayhem on IRC yesterday we decided its not worth implementing poetry, we wouldn't use most of its features and the fact we cannot overwrite the build script is somewhat stopping us at the moment. Maybe we can add this on a later stage.

Whats left are the pytest configurations, but thats left for a later work.

@bpepple thank you for starting the process, it was a huge help to have some kind of orientation.