Dev dependencies not separated

[bjoluc]

First of all, thanks for building this library! I was banging my head against a wall trying to find a simple-yet-powerful application config approach like node-config in the Python ecosystem.

Unfortunately, pip-installing config2 installs its development dependencies as well. As of 2020, I would suggest to simply replace the setuptools-setupextras-based approach with Poetry to reduce manual boilerplate code. I would be happy to submit a PR if you welcome this.

I also have a few more problems and solutions in mind. Let me know if you welcome contributions, and I will file them in separate issues. Cheers!