registry.py: migrate deprecated pkg_resources

[hseg]

As noted in https://setuptools.pypa.io/en/latest/pkg_resources.html, pkg_resources is deprecated. We were only using it for iter_entry_points anyway, which has been moved to importlib.metadata:entry_points

Changes proposed in this pull request:

• Replace pkg_resources:iter_entry_points by importlib.metadata:entry_points

@xlcnd

[KenjiTakahashi]

FYI: Since 3.12, pkg_resources is no longer part of stdlib and setuptools are not a part of the standard Python distribution either. Means that to be able to use isbnlib one needs to pip install setuptools first.

[pierreay]

Hello. Two years without any commit. Is isbnlib still maintained?