Move resource management to importlib.resources

[gnn]

Right now, resources are accessed via a package's __file__ or __path__ attribute. As of Python 3.7 there is a standardized way in which this can be handled in Python, namely via importlib.resources. I suggest dropping support for Python 3.6 and refactoring the code relying on __file__ and/or __path__ to use the new mechanism, but I'd like to hear other people's thoughts about this as well.