Implement first class exception support

[hynek]

Fixes #368

This adds first class exception support to attrs. While on the first sight it might make more sense to have an argument like exc=True or even have a separate decorator, I think in the ligtht of import attrs this approach is better, because – of course – auto_exc would be True by default in @attrs.define.

Let me know what you think or whether I've missed some subtleties.

Open Questions:

1. Is it a good idea to put attributes with defaults into args too?
2. Should we add init=False, default=NOTHING too?

The more I look at the code I feel like it should be all (including default=NOTHING) or nothing (only non-default).

[rouge8]

I think this has the same issue on Python 2.7 that I ran into on #217:

$ python
Python 2.7.15 (default, Jan 15 2019, 13:50:43)
[GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from copy import deepcopy
>>> import attr
>>> @attr.s(auto_exc=True)
... class Error(Exception):
...     x = attr.ib()
...
>>> deepcopy(Error('msg'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python@2/2.7.15_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 190, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/usr/local/Cellar/python@2/2.7.15_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 329, in _reconstruct
    y = callable(*args)
TypeError: __init__() takes exactly 2 arguments (4 given)

I can also get a weird TypeError:

>>> Error(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<attrs generated init 9b279913e79c1ceee0f98d8b928f7dd9eb2ee432>", line 3, in __init__
TypeError: 'int' object is not iterable

[hynek]

Ouch, the second one is embarrassing. The first one…is there something we can do? 🤷‍♂️

[rouge8]

For the first one, I add an __attrs_post_init__ that calls Exception.__init__(obj, *attr.astuple(obj)): #368 (comment) . So maybe you can do that in the generated __init__ when it's an Exception subclass?

[hynek]

Well, this looks much nicer. What do you think?

That also resolves my original questions. Only init=True goes into it – period.

[rouge8]

Nice! 🎉