Bug: dataclass with frozen and slots fails if defaults not provided

[guacs]

Description

The following will fail:

from dataclasses import dataclass
import msgspec


@dataclass(frozen=True, slots=True)
class Foo:
    foo_value: int = 0


foo = msgspec.convert({}, Foo) # fails

The traceback:

Traceback (most recent call last):
  File "~/foo.py", line 9, in <module>
    foo = msgspec.convert({}, Foo)
          ^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 4, in __setattr__
dataclasses.FrozenInstanceError: cannot assign to field 'foo_value'

If the value for foo_value is provided in the initial dictionary, there is no error. Also, the error is only raised if slots=True is present.

[jcrist]

Thanks for the excellent reproducible issue, this has been resolved in #569. I'll try to make a release in the next week or so.