Replace pkg_resource with importlib

`pkg_resources` module (distributed with setuptools) is deprecated, and users are encouraged to use `importlib.resources` and `importlib.metadata`. See https://setuptools.pypa.io/en/latest/pkg_resources.html

~~So we currently have undeclared dependency on setuptools (currently satisfied transitively via `dulwich` and `jsonschema`)~~ The setuptools dependency is now explicit, and should be removed once this is fixed.

We need to replace the following functions from the `pkg_resources` module:
- [ ] `resource_listdir`
- [ ] `resource_isdir`
- [ ] `resource_stream`
- [ ] `resource_string`

See [migration guide](https://importlib-resources.readthedocs.io/en/latest/migration.html#migration-guide)