frozen=True incompatible with cache_hash=True as of 19.1.0

[pganssle]

Prior to #489, this used to work:

import attr
import copy

@attr.s(frozen=True, cache_hash=True)
class FrozenWithCache:
    pass


if __name__ == "__main__":
    copy.deepcopy(FrozenWithCache())

The issue is that frozen is implemented by throwing an error in __setattr__, and clearing the hash cache calls setattr. I think this can be solved by using the same mechanism that _make_hash uses for frozen classes, but I have not tried implementing this before.

Another option is to not clear the hash cache on serialization for frozen classes.