autodoc_preserve_defaults=True does not work with slices in default argument

Describe the bug

When using the autodoc_preserve_defaults=True option, any slicing operator being present in the default arguments results in a warning

WARNING: Failed to parse a default argument value for <function func at 0x7f56f5fdc430>: Unable to parse Slice object

and a fully expanded default argument (as if autodoc_preserve_defaults was not set at all).

How to Reproduce

Dedicated minimum working example:

$ git clone https://github.com/dokempf/mwe_autodoc_preserve_defaults.git
$ cd mwe_autodoc_preserve_defaults
$ pip install -r requirements.txt
$ cd docs
$ make html

Expected behavior

Documentation should show the default argument including the slice operator.

Your project

https://adaptivefiltering.readthedocs.io/en/latest/#adaptivefiltering.DataSet.save_mesh

Screenshots

No response

OS

Linux

Python version

3.9.7

Sphinx version

4.2.0

Sphinx extensions

sphinx.ext.autodoc

Extra tools

No response

Additional context

No response

Start by running the linked minimum working example with pip install -r requirements.txt and make html, then inspect the sphinx.ext.autodoc handling involved in preserving default arguments. Done means the generated documentation retains slice expressions in defaults without emitting the reported warning.