Skip empty doctests, stop blank filtering DeprecationWarnings

[ev-br]

• Put back the skip of docstrings with no doctest examples: pytest does it, and there is no reason not to. If we don't skip them here, pytest --doctest-modules reports them as if they had +SKIP anyway.
• Stop filtering all DeprecationWarnings. This was a temp filter IIUC, and this is bad practice in general. Either rewrite the docstring to avoid deprecated stuff, or use a more granular filter in dt_context.user_context_mgr.

[Sheila-nk]

This seems pretty straightforward. Feel free to merge @ev-br 🥳

[ev-br]

Thanks Sheila