Add stubs for gunicorn

[eugeneliukindev]

Source: https://github.com/benoitc/gunicorn
PyPi: https://pypi.org/project/gunicorn/
Issue: benoitc/gunicorn#1393

[eugeneliukindev]

@srittau
During the changes I encountered the following problem:

Source code:

class NumWorkersChanged(Setting):
    def nworkers_changed(server, new_value, old_value):
        pass

    default = staticmethod(nworkers_changed)

The nworkers_changed method is used only as a pass through to the staticmethod function to set the default value.

When trying to write stubs:

def nworkers_changed(server: Arbiter, new_value: int, old_value: int | None) -> None: ... 

I get the following error from mypy:

gunicorn/config.pyi:846: error: Self argument missing for a non-static method (or an invalid type for self)  [misc]
        def nworkers_changed(server: Arbiter, new_value: int, old_value: int | None) -> None: ...
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

How can I properly create a stub so I don't have to write type: ignore[misc]? If I add the @staticmethod decorator, it won't match the original function signature, and stubtest will also complain.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/worker.py:38: error: Unused "type: ignore[misc, no-any-unimported]" comment  [unused-ignore]
+ aiohttp/worker.py:71:13: error: Statement is unreachable  [unreachable]
+ aiohttp/worker.py:71:13: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-unreachable for more info
+ aiohttp/worker.py:75:26: error: Too few arguments  [call-arg]
+ aiohttp/worker.py:75:26: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-call-arg for more info
+ aiohttp/worker.py:75:26: note: "wsgi" is considered instance variable, to make it class variable use ClassVar[...]
+ aiohttp/worker.py:117: error: Unused "type: ignore" comment  [unused-ignore]