Cattrs

- Python 3.8 support.

- Python 3.7 support.

- The disambiguation function generator now supports unions of _attrs_ classes and NoneType.

- Distribution fix.

- Removed the undocumented `Converter.unstruct_strat` property setter.
- Removed the ability to set the `Converter.structure_attrs` instance field.
- Some micro-optimizations were applied; a `structure(unstructure(obj))` roundtrip
is now up to 2 times faster.

- Packaging fixes.
([17](https://github.com/python-attrs/cattrs/pull/17))