move to importlib

[nkrabben]

small change to move away from the deprecated pkg_resources

importlib.metadata is not supported in 3.7, which is still supported by bagit-python.

The library also doesn't have an exception class but it can return None.

[acdha]

Python 3.7's end of life was in June so maybe the answer here is to mark that as no longer supported?

[branchvincent]

This would be great to merge/release as Python 3.12 no longer comes bundled with setuptools (and thus pkg_resources): python/cpython#95299

[sevein]

@nkrabben, I believe the issue has been resolved in #172. Can we close this?