Use importlib to handle version and package resources, where available

[peterbarker]

We fall back to the previous efforts on failure.

Tested back to 3.6

2.7 was a complete write-off due to pre-existing changes.

[peterbarker]

Rejig this to just use importlib on Windows

[peterbarker]

Since pkg_resources is going away and importlib has worked since forever, move to just using it now.