This replaces the whole usage of the separate package
`sphinx-autodoc-typehints` with the built-in support from `autodoc`,
available (with this combination of options) since Sphinx 5.0.  This
removes a docs dependency that has caused us significant problems before
due to its instability, and the built-in handling in `autodoc` is much
cleaner as well; rather than trying to textually modify the docstring,
this version (correctly) mutates the parsed rST structure, which also
naturally it 100% compatible with the Napoleon processing.

…pehints