CLN: tighten cython declarations
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

@jbrockmendel just fyi in case it's what you're troubleshooting, have run into the annoying `ModuleNotFoundError: No module named 'pandas._libs.interval'` error before - think it's usually caused by a failure in the build step, not an actual import problem > think it's usually caused by a failure in the build step, not an actual import problem

you're right.  unfortunately the message we re-raise in `__init__` obscures any actually-helpful information in the original exception message, hence removing it for the moment