Sphinx raises warning about custom typehints_formatter

[jl-wynen]

Since Sphinx v7.3.0, there is a warning about config values that are not pickleable. When building with warning_is_error = True, this fails the build. See sphinx-doc/sphinx#12300 for context.

For sphinx-autodoc-typehints, this means that a custom typehints_formatter leads to a build failure.

To reproduce, simply use this conf.py:

project = 'custom_formatter'
extensions = [
    'sphinx.ext.autodoc',
    'sphinx_autodoc_typehints',
]
warning_is_error = True
show_warning_types = True

def formatter(annotation, config):
    return None

typehints_formatter = formatter

It results in this warning:

sphinx.errors.SphinxWarning: cannot cache unpickable configuration value: 'typehints_formatter' (because it contains a function, class, or module object) [config.cache]

Apart from triggering a warning that needs to be silenced, this also prevents caching. So it would be great to have a solution that avoids specifying a callable as typehints_formatter. One option would be using a fully qualified path and make sphinx-autodoc-typehints import a function from that path. E.g. typehints_formatter = 'mypackage.sphinxext.typehints_formatter'. See also this comment: sphinx-doc/sphinx#12300 (comment)