Replace deprecated pkg_resources with importlib.metadata

[onixlas]

The current implementation uses pkg_resources.get_distribution() to get the package version, which is now deprecated as officially stated in the setuptools documentation:

Use of pkg_resources is deprecated in favor of importlib.resources, importlib.metadata and their backports (importlib_resources, importlib_metadata). Some useful APIs are also provided by packaging (e.g. requirements and version parsing). Users should refrain from new usage of pkg_resources and should work to port to importlib-based solutions.

I've replaced pkg_resources with Python's standard library alternative:

from importlib.metadata import version, PackageNotFoundError

try:
    __version__ = version("rrcf")
except PackageNotFoundError:
    __version__ = "unknown"

Thank you for your work on rrcf, it's a great package