Describe using importlib-metadata as an alternative to pkg_resources for single-sourcing versions

[jwodder]

Item 5 in "Single-sourcing the package version" currently presents its approach using pkg_resources. However, pkg_resources is inefficient (and possibly deprecated?), and more efficient & modern alternatives to it are being developed in the form of importlib.resources and importlib.metadata. This PR extends item 5 to also show how to achieve the same thing using importlib.metadata. It also adds a note that the pkg_resources version requires setuptools as a runtime dependency.

I expect there will be some debate over whether to present both methods side by side or whether to delete the pkg_resources method in favor of importlib.metadata. I'm open to both possibilities.

[xavfernandez]

I think the importlib.metadata should be presented before the pkg_resources one.

[pfmoore]

I agree with @xavfernandez, I'd be in favour of putting the importlib.metadata approach first.

[jwodder]

@xavfernandez @pfmoore importlib-metadata section moved before the pkg_resources section.

[pradyunsg]

Thanks for the PR @jwodder, and the reviews @pfmoore and @xavfernandez!