Use importlib.resources instead of pkg_resources

[muttener]

This fixes #33

The tests are succeeding and I tested coverage-badge locally (on linux) by running:

for version in 3.10 3.11 3.12 3.13 3.14; do uv run --python $version --with . coverage-badge -o coverage-$version.svg; done

The double joinpath in the implementation is needed to support python 3.10, 3.11 and 3.12.