Describe the bug
Positional-only arguments are mishandled in signatures with / always being put at the start of a signature with positional-only arguments regardless of how many arguments are positional only.

To Reproduce
With the following test code being targeted by mkdocstrings

# test.py
def bar(foo: str, baz: int, /, *, bar: int = 123) -> None:
    """meow"""

and the following mkdocs.yml

# mkdocs.yml
site_name: "Docs"
plugins:
  - search
  - mkdocstrings:
      default_handler: python
      watch:
        - test.py

build documentation from this module and watch it place / at the start of bar's signature as shown in the following screenshot.

Expected behavior
/ should appear after baz in the signature.

Screenshots

System (please complete the following information):

• mkdocstrings-python version: 0.7.0
• mkdocstrings version: 0.19.0
• griffe version: 0.20.0
• mkdocs version: 1.3.0
• Python version: 3.10
• OS: Windows

Additional context
This leads to an error being raised in cases where black is used to reformat function signatures with positional only arguments as / cannot be the first argument of a function syntax wise.

I can't really read this code but from what I can tell the responsible code might be here

(where it's adding in / the moment it finds a positional only argument that isn't named self or cls rather than waiting until it reaches the end of the positional only arguments). I did try griffe with the example code and it seemed to be handling positional-only arguments properly.