Implement first class exception support (#500)

* Implement first class exception support

Fixes #368

* Ensure single-attrib classes work too

cf https://github.com/python-attrs/attrs/pull/500#pullrequestreview-201913569

* Call into BaseException to initialiaze self.args

* Leave __str__ alone since we upcall

Based on Python pizza hallway feedback by @ambv.

* remove stray stage

* nope
7 files changed