Pydantic `frozen=False` override does not make subclass fields writable

The following write to the `Child.text` attribute is incorrectly rejected by ty:

```py
from pydantic import BaseModel, ConfigDict

class Frozen(BaseModel):
    model_config = ConfigDict(frozen=True)

class Mutable(Frozen):
    model_config = ConfigDict(frozen=False)

class Child(Mutable):
    text: str

child = Child(text="before")
child.text = "after"  # Should be accepted.
```