Replace use of pkg_resources with importlib

[WilliamJamieson]

Replaces all uses of pkg_resources with importlib throughout asdf.

Fixes #941

[zacharyburnett]

This looks good; should we also add importlib_metadata to the build configuration as a dependency contingent on Python version?

[WilliamJamieson]

This looks good; should we also add importlib_metadata to the build configuration as a dependency contingent on Python version?

We already are for other uses:

asdf/pyproject.toml

Line 19 in d54afa2

'importlib_resources >=3; python_version <"3.9"',

[WilliamJamieson]

This looks good; should we also add importlib_metadata to the build configuration as a dependency contingent on Python version?

Turns out I was wrong, we do have to add importlib-metadata as a dependency for python 3.9 or older. Why do they all have to be named similar things?

[WilliamJamieson]

Note "Required" needs to be updated after this is merged due to the CI job name changes.

[braingram]

Looks good to me.