`importlib.resources.read_text` is deprecated

I saw this log snippet go by when running tests:

```
.../site-packages/case_utils/ontology/__init__.py:43: DeprecationWarning: read_text is deprecated. Use files() instead. Refer to https://importlib-resources.readthedocs.io/en/latest/using.html#migrating-from-legacy for migration advice.
```