Add importlib.metadata as an approach in "Single-sourcing the package version"

[chrahunt]

The current version of "Single-sourcing the package version" has an example using pkg_resources. It would probably make sense to include an item using importlib.metadata since it will be standard in 3.8+ with a backport as importlib_metadata and can serve the same purpose. That may look like

from importlib_metadata import version

__version__ = version(__package__)

[soxofaan]

I think this issue can be closed as it is already handled in 3c7c07f (and followup tweaks)

[bhrutledge]

I think the documentation could be improved; looking at option 5 in https://packaging.python.org/en/latest/guides/single-sourcing-package-version/, I don't actually see a complete example of this technique.

[pradyunsg]

I suggest that we close this out and file a separate issue for improving the option 5 to de-emphasize pkg_resources and prefer importlib.metadata on 3.8+.

[ncoghlan]

Resolved in #1612