If you define a frozen dataclass with slots and deep copy it, an error will occur. If you run the same code and remove the slots, the error will not occur. I assume this behavior is not intentional? Apologies if I'm submitting this wrong, this is the first time I've submitted an issue here so I'm not quite sure how to do it properly.

Example below:

```
from dataclasses import dataclass
from copy import deepcopy

@dataclass(frozen=True)
class FrozenData:
    # Without slots no errors occur?
    __slots__ = "my_string",

    my_string: str

deepcopy(FrozenData(my_string="initial"))
```

Error that occurs:
```
dataclasses.FrozenInstanceError: cannot assign to field 'my_string'
```