Cattrs

- _cattrs_ now has a benchmark suite to help make and keep cattrs the fastest it can be. The instructions on using it can be found under the [Benchmarking](https://cattrs.readthedocs.io/en/latest/benchmarking.html) section in the docs.
([123](https://github.com/python-attrs/cattrs/pull/123))
- Fix an issue unstructuring tuples of non-primitives.
([125](https://github.com/python-attrs/cattrs/issues/125))
- _cattrs_ now calls `attr.resolve_types` on _attrs_ classes when registering un/structuring hooks.
- `GenConverter` structuring and unstructuring of _attrs_ classes is significantly faster.

- `converter.unstructure` now supports an optional parameter, `unstructure_as`, which can be used to unstructure something as a different type. Useful for unions.
- Improve support for union un/structuring hooks. Flesh out docs for advanced union handling.
([115](https://github.com/python-attrs/cattrs/pull/115))
- Fix `GenConverter` behavior with inheritance hierarchies of _attrs_ classes.
([117](https://github.com/python-attrs/cattrs/pull/117 [#116](https://github.com/python-attrs/cattrs/issues/116>))
- Refactor `GenConverter.un/structure_attrs_fromdict` into `GenConverter.gen_un/structure_attrs_fromdict` to allow calling back to `Converter.un/structure_attrs_fromdict` without sideeffects.
([118](https://github.com/python-attrs/cattrs/issues/118))

- The default disambiguator will not consider non-required fields any more.
([108](https://github.com/python-attrs/cattrs/pull/108))
- Fix a couple type annotations.
([107](https://github.com/python-attrs/cattrs/pull/107) [#105](https://github.com/python-attrs/cattrs/issues/105))
- Fix a `GenConverter` unstructuring issue and tests.

- 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.