CLN: tighten cython declarations

[jbrockmendel]

• closes #xxxx
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

[mzeitlin11]

@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

[jbrockmendel]

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