Describe the bug
When a class inherits another class from a mocked import as defined in autodoc_mock_imports, the docstring is skipped by autodoc when building documentation.

To Reproduce
Example project taken from #4965

Project module foo.py:

import bar

class FooClass(bar.BarClass):
    """This is myclass"""
    pass

conf.py:

import sys
import os

autodoc_mock_imports = ['bar']

sys.path.insert(0, os.path.abspath('.'))

extensions = [
    'sphinx.ext.autodoc',
]

contents.rst:

foo
===

.. automodule:: foo
    :members:
    :undoc-members:
    :show-inheritance:

Build Command

$ sphinx-apidoc -f -o docs/ .
$ sphinx-build -b html docs/ docs/build/html -E -a

Expected behavior
The built documentation would include the foo.FooClass class and its docstring.

Your project
mocked_import_issue.zip

Environment info

• OS: Linux
• Python version: 3.8.5
• Sphinx version: 3.2.1
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools:

Additional context
Sphinx 3.0.1 build correctly, later versions skip the foo.FooClass when building documentation.