You probably have to override the signature by hand in the reST file.
It’s hard to come up with a better answer. Autodoc imports the modules it documents, so all module-level code (including default function arguments) is executed.
See also these similar questions: here and here.
I just realized that there is another option. You can override the signature by including it as the very first line of the docstring. See the documentation of the autodoc_docstring_signature configuration variable, and this answer.