msgspec.defstruct is not compatible with msgspec.StructMeta

Description

Structs that were defined with custom metaclass (subclass of msgspec.StructMeta) can't be subclassed with msgspec.defstruct. (msgspec 0.20.0)

Following code:

import msgspec

class _FooStructMeta(msgspec.StructMeta):
    def __new__(mcls, name, bases, namespace, **struct_config):
        return super().__new__(mcls, name, bases, namespace, **struct_config)

class FooStruct(msgspec.Struct, metaclass=_FooStructMeta):
    pass

BarStruct = msgspec.defstruct(name="BarStruct", fields=[("id", int)], bases=(FooStruct,))

Raises this error:

Traceback (most recent call last):
  File "/home/jirka642/.config/JetBrains/PyCharm2025.3/scratches/scratch_1.py", line 13, in <module>
    BarStruct = msgspec.defstruct(name="BarStruct", fields=[("id", int)], bases=(FooStruct,))
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jirka642/.config/JetBrains/PyCharm2025.3/scratches/scratch_1.py", line 6, in __new__
    return super().__new__(mcls, name, bases, namespace, **struct_config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Struct types cannot define __slots__

I have been able to hotfix it by running namespace.pop("__slots__", None) in the metaclass, but I'm not sure if that is safe.

Start with the provided Python reproduction and the msgspec.defstruct entry point, then inspect how its bases and metaclass are handled. Done means the reproduction creates BarStruct successfully when FooStruct uses a custom StructMeta, with coverage for the reported case.