importlib: prefer importlib.resources over importlib_resources

[askervin]

Using pepc simply by cloning the repo and running pepc fails on importing importlib_resources. Looking at the code, this could be avoided without installing the importlib_resources dependency in recent Python 3 versions, simply by using importlib.resources.

According to the docs, importlib.resources.files got added to the API in Python 3.9, and the only change so far, that is in Python 3.12, does not affect this code.

Therefore I'd propose getting rid of this minor inconvenience by trying to use importlib.resources.file, and only fallback to importlib_resources.files in case importlib.resources.file is not found in the API (Python older than 3.9).

I'm still leaving the TODO comment untouched as a reminder to get rid of the importlib_resources dependency when dropping support of outdated Python 3.

I tested this change with Python 3.13.5 and Python 3.9.23. They both worked fine, and they both successfully used importlib.resources.files.