[TODO] Stop importing `pkg_resources`

It's deprecated and causes the following warning:
```console
E   DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('sphinxcontrib')`.
E   Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
```
