Now python domain supports the "~" prefix at the beginning of the
typehints of the function signatures:

  .. py:function:: func(x: ~typing.Dict)

It's rescognized as the same as python reference roles do
(ex. :py:class:`~typing.Dict`).

To make the generated function signatures simple, this adds a new
parameter `smartref` to sphinx.util.typing:stringify() to suppress the
leading module name from typehints.

To make the generated function signatures simple, this adds a new
parameter `unqualified_typehints` to sphinx.util.inspect:
stringify_signature() to suppress the leading module name of
typehints.

…fied_typehints

If autodoc_unqualified_typehints feature enabled, autodoc suppresses the
leading module names of typehints of function signatures (ex.
`io.StringIO` -> `StringIO`)