Allow Poetry to select best Python version

[BrandonLWhite]

Resolves: #76

This PR alters the VenvBundle virtual environment creation to leverage Poetry's ability to select the "best" Python version to use based upon the pyproject's python dependency specification.

• A subclass of EnvManager is used to override the "in_project_venv" methods as a way to leverage the Poetry behavior we need to (re)create a venv at an arbitrary location.
• All checking of existing venv is now done in Poetry, and so code related to that is removed from VenvBundle
• InvalidCurrentPythonVersionError exception handling is used to deal with existing venv that is incompatible. Poetry raises an exception in this case, so the solution I implemented to support this use case is to call create_venv again with force=True

[radoering]

LGTM. 👍

Just updated the readme + some minor coding style changes.