Sphinx build stuck in reading sources... when `singledispatch` and `autodoc_mock_imports` are used

Describe the bug

Sphinx build hangs forever (until it times out) if autodoc_mock_imports is used with code that uses singledispatch.

How to Reproduce

See this repository and its GitHub Actions to reproduce the issue:
https://github.com/rozsasarpi/sphinx_singledispatch

• stuck build (timed out) when singledispatch and autodoc_mock_imports are both used: https://github.com/rozsasarpi/sphinx_singledispatch/runs/6999430788?check_suite_focus=true#step:5:15
• working build without autodoc_mock_imports: https://github.com/rozsasarpi/sphinx_singledispatch/runs/6999461954?check_suite_focus=true
• working build without singledispatch: https://github.com/rozsasarpi/sphinx_singledispatch/runs/6999520591?check_suite_focus=true

Expected behavior

make html finishes the build.

Your project

https://github.com/rozsasarpi/sphinx_singledispatch

Screenshots

OS

Linux/Win

Python version

3.8, 3.9

Sphinx version

5.0.2/4.5.0

Sphinx extensions

sphinx.ext.autodoc, myst_parser

Extra tools

No response

Additional context

No response

Start with the linked sphinx_singledispatch reproduction repository and run its make html build using both singledispatch and autodoc_mock_imports. Compare it with the working builds that omit one of those features, then trace the sphinx.ext.autodoc processing involved. Done means the combined configuration completes instead of hanging while reading sources.