Switch to poetry and add community guidelines

[c00kiemon5ter]

With this changeset we are moving away from setuptools and start using poetry for development and release management. Support is added through pyproject.toml for supporting tools for development, testing and documentation.

At the same time community guidelines are added to explain the policies and workflows around the project.
More changes will come for:

• development guidelines (how to use poetry and setup a local development environment)
• updated readme

and later:

• updated changelog format (https://keepachangelog.com)
• commit messages guidelines
• updated docs (switch to mkdocs and markdown)
• task management (Makefile or taskipy)

PS: The current poetry.lock has been generated by installing the dev, test, coverage and docs groups.
PS2: The next version is set to 7.3.0-alpha.

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?