Class with only `__init_subclass__` raises `attr-defined`

Hello! I dug around the issue tracker to try and find any references to this kind of issue, but didn't find anything relevant. Apologies if this is a duplicate, but it seemed like it was unique enough for a report.

Bug Report

When adding a type hint to a mixin class's function, an error is raised. Previously, the function was untyped so was not being evaluated (i.e. not in --strict mode).

To Reproduce
I boiled it down to a simpler example. The contents of main.py:

class HasEvents:
    def __init_subclass__(cls, /, **kwargs):
        super().__init_subclass__(**kwargs)
        cls.foo = []  # this type doesn't appear to matter
        return cls
    
    def record_event(self, *, some_other_arg: str):
        event = self.foo.append(some_other_arg)  # <== error here
        # do something else with `event`...
        return event

https://mypy-play.net/?mypy=1.2.0&python=3.11&gist=90a1e568fd9630201f21c28f94167138
https://gist.github.com/mypy-play/90a1e568fd9630201f21c28f94167138

Actual Behavior

main.py:8: error: "HasEvents" has no attribute "Event"  [attr-defined]

I found that changing __init_subclass__ to __init__ passes.

Your Environment

I tried a variety of Python and mypy versions from the playground to see if it was new, and went back to Python 3.7 and mypy 0.750 - so versions, flags and command line options have been isolated from the equation.

Pull request part of the greater project, where I disabled the check: pypi/warehouse@bc76ecf (#13593)

Inizia con la riproduzione minima in main.py e con il comportamento di init_subclass quando viene verificato self.foo in record_event. Traccia il percorso rilevante del controllo dei tipi e confrontalo con la variante funzionante con init. Il lavoro è completato quando il caso riprodotto non segnala più un errore attr-defined e i controlli esistenti rimangono corretti.