Issue 43999: Cannot pickle frozen dataclasses with slots

Originally reported in https://github.com/ericvsmith/dataclasses/issues/154

import pickle
from dataclasses import dataclass

@dataclass(frozen=True, slots=True)
class ExampleDataclass:
    foo: str
    bar: int



assert ExampleDataclass.__slots__ == ("foo", "bar")

assert pickle.loads(
    pickle.dumps(ExampleDataclass("a", 1))
) == ExampleDataclass("a", 1)

  File "<string>", line 4, in __setattr__
dataclasses.FrozenInstanceError: cannot assign to field 'foo'

I'll get a PR ready, likely based on ariebovenberg's solution in the above mentioned issue.