`c.__subclasses__(c)` requires explicit self as input argument when `c` inherits from `StructMeta`

[mcrumiller]

Description

Calling __subclasses__() to get the list of subclasses of a class derived from StructMeta requires that the class be explicitly passed into the function call, which is a bit redundant:

from abc import ABC
from msgspec import Struct, StructMeta

class A1(ABC): ...
class C1(Struct): ...
class C2(StructMeta): ...

A1.__subclasses__()    # OK
C1.__subclasses__()    # OK
C2.__subclasses__()    # ERROR: Argument missing for parameter "self"
C2.__subclasses__(C2)  # OK

[mcrumiller]

I realize now that perhaps StructMeta is not meant to be directly inherited, and instead used with class C2(metaclass=StructMeta), so perhaps this should be closed/is not an error?