Switch to poetry and tox

[nsorros]

We want to remove the need to maintain requirements and setup.py dependencies so we want to introduce poetry and treat this as a library and not an application to make it easier to maintain. We want to keep the distinction between inference (default) and development where all deps will be installed.

We also want to switch to testing with tox as a better way to test the library because of the isolated install and remove the need for pytest. At the moment we have a setup for both pytest and tox.