Add support for resources in namespace packages

[FFY00]

Fixes #68

[anthrotype]

I don't know how the this project exactly relates to the built-in importlib.resources (I see in the README that you recommend to use the buillt-in module for Python >= 3.9), but I wonder: does the built-in importlib.resources in Python 3.9 also supports namespace packages? Or do I need to use this importlib_resources "backport" to get this feature even on Python 3.9? (which would make the backport more advanced the the real thing). Thanks

[jaraco]

This project and importlib.resources are synchronized. Commits here are ported to CPython and vice-versa. The namespace package support was only recently added and hasn't yet been ported to CPython, so if you want to support namespace packages, you'll want to rely on importlib_resources on CPython 3.9 or earlier. Most likely, the namespace package support will land in Python 3.10.

[anthrotype]

Makes sense, thank you for the clarification