New autodoc_class_signature config option gives an exception

[miohtama]

Describe the bug

The option causes an exception on build when used as documented

How to Reproduce

When setting autodoc_class_signature = "separated" in conf.py

I get:

Exception occurred:
  File "/Users/moo/Library/Caches/pypoetry/virtualenvs/capitalgram--bm4msE3-py3.8/lib/python3.8/site-packages/sphinx/ext/autodoc/__init__.py", line 1452, in __init__
    self.options.special_members.append('__new__')
AttributeError: 'set' object has no attribute 'append'

Expected behavior

The signature setting works or explains if there is something wrong with the configuration file.

Your project

https://github.com/miohtama/capitalgram-onchain-dex-quant-data/blob/master/docs/source/conf.py

Screenshots

No response

OS

Mac

Python version

3.0

Sphinx version

4.1.1

Sphinx extensions

sphinx.ext.autodoc

Extra tools

No response

Additional context

Probably this bit is faulty - cannot decide if the option is set or list

    def __init__(self, *args: Any) -> None:
        super().__init__(*args)

        if self.config.autodoc_class_signature == 'separated':
            # show __init__() method
            if self.options.special_members is None:
                self.options['special-members'] = {'__new__', '__init__'}
            else:
                self.options.special_members.append('__new__')
                self.options.special_members.append('__init__')

        merge_members_option(self.options)