The default value of autodoc_typehints_format configuration is changed
to `'smart'`. It will suppress the leading module names of typehints
(ex. `io.StringIO` -> `StringIO`).
refs: #9075
Add a new extension `sphinx.ext.autodoc.preserve_defaults`.
It preserves the default argument values of function signatures in source code
and keep them not evaluated for readability. This is an experimental
extension and it will be integrated into autodoc core in Sphinx-4.0.