Update to modern Python packaging structure

[AKuederle]

To simplify developing and maintaing the package, I would suggest to switch to a more modern package structure/tool chain.

My suggestioin would be to:

• switch to poetry as dependency manager and build tool
• add some github workflows to auto publish
• Switch to pytest instead of nosetest

@friggog @lmjohns3 any opinions on that?

[friggog]

Haven't used poetry, but not against it - certainly in favour of workflows and pytest!

[AKuederle]

Poetry merged in #40

For further major changes (e.g. linting/code formatting/pytest) I want to wait unitl the change sets from #33 are mostly merged so that we don't run into merge conflicts constantly because of formatting changes.