Cattrs

- Add metadata for supported Python versions.
([103](https://github.com/python-attrs/cattrs/pull/103))

- Python 2, 3.5 and 3.6 support removal. If you need it, use a version below 1.1.0.
- Python 3.9 support, including support for built-in generic types (`list[int]` vs `typing.List[int]`).
- _cattrs_ now includes functions to generate specialized structuring and unstructuring hooks. Specialized hooks are faster and support overrides (`omit_if_default` and `rename`). See the `cattr.gen` module.
- _cattrs_ now includes a converter variant, `cattr.GenConverter`, that automatically generates specialized hooks for attrs classes. This converter will become the default in the future.
- Generating specialized structuring hooks now invokes [attr.resolve_types](https://www.attrs.org/en/stable/api.html#attr.resolve_types) on a class if the class makes use of the new PEP 563 annotations.
- _cattrs_ now depends on _attrs_ >= 20.1.0, because of `attr.resolve_types`.
- Specialized hooks now support generic classes. The default converter will generate and use a specialized hook upon encountering a generic class.

- _attrs_ classes with private attributes can now be structured by default.
- Structuring from dictionaries is now more lenient: extra keys are ignored.
- _cattrs_ has improved type annotations for use with Mypy.
- Unstructuring sets and frozensets now works properly.

- Python 3.8 support.

- Python 3.7 support.

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