Use importlib.resources instead of deprecated pkg_resources

[Rubo3]

This patch fixes the warnings I get when running the tests, by using importlib.resources.files instead of pkg_resources.resource_filename. All of them pass.

[tybug]

this raises our minimum python version to 3.9 - is there an alternative which is still more modern than pkg_resources? Which I agree needs to go.

[Rubo3]

Since you look for files in the same directory of the module, what about

from pathlib import Path
...
    return CollectionDB.from_path(Path(__file__).parent / name)

pathlib was introduced in Python 3.4.

[tybug]

huh, I assumed there was some reason why a package-relative path was necessary...but you're right pathlib seems to work fine here. Not sure why that wasn't used from the beginning.

[Rubo3]

Please tell me when you're ready to merge this PR. Thanks.

[Rubo3]

Thank you!