Migrate to importlib.metadata.

[emollier]

As initially identified in Debian bug #1083383, Fastaq depends on the Python module pkg_resources, deprecated since python3.11 and obsoleted in python3.13, in favor of importlib.metadata, available since python3.8.

In case Fastaq still needs to cover ancient Python interpreter versions preceeding 3.8, the change also includes a fallback to pkg_resources if the importlib is not located properly.