References to builtins not working in autodoc class aliases

[timobrembeck]

Describe the bug
When aliasing an attribute to another class, sphinx adds the line "alias of ..." to the attribute's documentation.
If the other class is a builtin, this adds e.g.:

alias of :class:`builtins.dict`

However, the intersphinx mapping to the official Python documentation only works without the "builtins"-part in the module path.

When using the nit-picky mode of sphinx, this results in the following warning:

WARNING: py:class reference target not found: builtins.int

To Reproduce
There's a test case which shows this behaviour.

Expected behavior
I expect the autodoc extension to generate the correct links suited for intersphinx mappings:

alias of :class:`dict`

Environment info

• OS: Arch Linux
• Python version: 3.7.9 & 3.8.6
• Sphinx version: 3.3.1
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: None

Additional context
I think it makes sense to use the utilitiy function restify for this. I will provide a patch soon.