Switch to using importlib.metadata

[s-t-e-v-e-n-k]

pkg_resources is a little heavyweight, and has some rather glaring shortcomings for checking the metadata of an install Python package. importlib.metadata is contained in the standard library (as of 3.8), and provides a much nicer interface. Switch to using it, falling back to importlib_metadata if the Python version is too old.

[jquast]

Thanks for this, this all seems sensible. I will have this PR merged for next release with #61

I was just working with pkg_resources in my day job so I appreciate you teaching me about this importlib.metadata :)