Autodoc: Method signature from docstring does not work for methods with no additonal arguments

Method signatures are not inferred from docstring, if no arguments are given.

Problem

I have a bunch of types created using C-extensions. I would like to include their function signature using the autodoc_docstring_signature option. For most methods, this works correctly. However, for functions accepting no additional arguments (other than self), the signature is simply

"""
some_method()

The description begins here.
...
"""

which renders as

some_method

instead of the expected

some_method()

This problem does only occur in methods from C-extensions, methods in normal Python classes are rendered correctly.

Reproducible project / your project

https://github.com/padix-key/biotite branch doc

For example in the type biotite.structure.BondList in the method as_array()

Environment info

• OS: Xubuntu 18.04
• Python version: 3.6
• Sphinx version: 1.7.8

Start with the autodoc_docstring_signature handling and reproduce the issue using biotite.structure.BondList.as_array() from the linked project. The work is complete when a C-extension method documented as some_method() renders with the parentheses instead of only some_method.