frozen=True doesn't play well with converters

[Deewiant]

Code that I would expect to run without errors:

import attr

@attr.s(frozen=True)
class Ice:
    x = attr.ib(convert=lambda y: y + 1)

assert Ice(1).x == 2

Actual runtime result:

Traceback (most recent call last):
  File "asdf.py", line 7, in <module>
    assert Ice(1).x == 2
  File "<attrs generated init cd74cad05a4efad0b01528e489d51126583d23cb>", line 4, in __init__
  File "/usr/lib/python3.5/site-packages/attr/_make.py", line 483, in _convert
    setattr(inst, a.name, a.convert(getattr(inst, a.name)))
  File "/usr/lib/python3.5/site-packages/attr/_make.py", line 156, in _frozen_setattrs
    raise FrozenInstanceError()
attr.exceptions.FrozenInstanceError

Clearly _convert isn't aware of the class's frozenness. I took a quick peek at the code and it seems like it should perhaps be using _cached_setattr instead of the ordinary setattr?

[Tinche]

Roger, I can put together a fix in the evening.