Manage dependencies in a venv, with poetry (WIP)

[apehex]

Simplify the devops thanks to poetry:

• reproducible dev environment
• version tagging from a single file, without the need for 'bump2version'
• tidies the repo, merging files with duplicate settings (ie the awful 'requirements.txt', 'setup.py' and 'setup.cfg')
• a single command instead of virtualenv + pip / pipenv

Deals with issues #445, #585.
Overrides PRs #524 and #519.

Only affects the repository slug.
The same could be done with the 'cookiecutter-pypackage' parent.

Let in some fresh air, merge this please!
Thanks