Replace deprecated use of pkg_resources

[Mark Piper]

A warning about using pkg_resources appears in the tests:

/Users/mpiper/projects/standard_names/standard_names/__init__.py:2:
  DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    import pkg_resources

The info in the link in the warning message suggests moving to importlib.metadata.

[Mark Piper]

It looks like the package is importlib-metadata (docs and on PyPI).