Add testing configuration for running pytest locally

[rossbar]

#132 added notebook smoketesting with pytest-nbval to CI, but didn't add any pytest configuration nor a way to automatically handle installing testing dependencies. Handling this configuration correctly will make it easier for users to smoketest notebooks locally with pytest. #139 is related as it should also be handled via pytest config. One idea would be to use tox.ini - see #132 (comment).