tox: fix poetry usage

Currently we are using tox, a virtual environment orchestrator, to run an external poetry command, which has its own virtual environment. This makes tests slow, as we don't get tox's optimizations, and we are provisioning two virtual environments, one in tox and one in poetry.

This patch remove the external poetry usages and makes use of tox as it was designed. It uses the tox-poetry-dev-dependencies[1] plugin, which does not need to be installed on the host, to install the dev dependencies, as for some reason poetry does not export these via an extra, as other backends normally do. The tests are now much faster, as we skip provisioning the poetry environment entirely, and we make use of tox's caching and other optimizations.

[1] https://github.com/sinoroc/tox-poetry-dev-dependencies

Signed-off-by: Filipe Laíns lains@riseup.net