Poetry package management, basic github action, isort

[tony]

To put #2 into more manageable chunks, this adds poetry package management and pinning for python 3.x

It also adds isort, together with black this can be used as make black isort or run automatically within your editor.

In this PR I also include sorting of imports and run black on the whole python codebase (files missed from #1)

[tony]

@VladimirParkhomenko Thoughts on this? It adds:

• poetry for package management
• isort for import sorting
• and a boilerplate github action to lint with flake8