Cattrs

- Fix `GenConverter` mapping structuring for unannotated dicts on Python 3.8.
([151](https://github.com/python-attrs/cattrs/issues/151))
- The source code for generated un/structuring functions is stored in the `linecache` cache, which enables more informative stack traces when un/structuring errors happen using the `GenConverter`. This behavior can optionally be disabled to save memory.
- Support using the attr converter callback during structure.
By default, this is a method of last resort, but it can be elevated to the default by setting `prefer_attrib_converters=True` on `Converter` or `GenConverter`.
([138](https://github.com/python-attrs/cattrs/issues/138))
- Fix structuring recursive classes.
([159](https://github.com/python-attrs/cattrs/issues/159))
- Converters now support un/structuring hook factories. This is the most powerful and complex venue for customizing un/structuring. This had previously been an internal feature.
- The [Common Usage Examples](https://cattrs.readthedocs.io/en/latest/usage.html#using-factory-hooks) documentation page now has a section on advanced hook factory usage.
- `cattr.override` now supports the `omit` parameter, which makes _cattrs_ skip the atribute entirely when unstructuring.
- The `cattr.preconf.bson` module is now tested against the `bson` module bundled with the `pymongo` package, because that package is much more popular than the standalone PyPI `bson` package.

- `Literal` s are not supported on Python 3.9.0 (supported on 3.9.1 and later), so we skip importing them there.
([150](https://github.com/python-attrs/cattrs/issues/150))

- `cattr.global_converter` (which provides `cattr.unstructure`, `cattr.structure` etc.) is now an instance of `cattr.GenConverter`.
- `Literal` s are now supported and validated when structuring.
- Fix dependency metadata information for _attrs_.
([147](https://github.com/python-attrs/cattrs/issues/147))
- Fix `GenConverter` mapping structuring for unannotated dicts.
([148](https://github.com/python-attrs/cattrs/issues/148))

- _cattrs_ now uses Poetry.
- `GenConverter` mapping structuring is now ~25% faster, and unstructuring heterogenous tuples is significantly faster.
- Add `cattr.preconf`. This package contains modules for making converters for particular serialization libraries. We currently support the standard library `json`, and third-party `ujson`, `orjson`, `msgpack`, `bson`, `pyyaml` and `tomlkit` libraries.

- Fix an issue with `GenConverter` unstructuring _attrs_ classes and dataclasses with generic fields.
([65](https://github.com/python-attrs/cattrs/issues/65))
- `GenConverter` has support for easy overriding of collection unstructuring types (for example, unstructure all sets to lists) through its `unstruct_collection_overrides` argument.
([137](https://github.com/python-attrs/cattrs/pull/137))
- Unstructuring mappings with `GenConverter` is significantly faster.
- `GenConverter` supports strict handling of unexpected dictionary keys through its `forbid_extra_keys` argument.
([142](https://github.com/python-attrs/cattrs/pull/142))

- Fix an issue with `GenConverter` un/structuring hooks when a function hook is registered after the converter has already been used.
- Add support for `collections.abc.{Sequence, MutableSequence, Set, MutableSet}`. These should be used on 3.9+ instead of their `typing` alternatives, which are deprecated.
([128](https://github.com/python-attrs/cattrs/issues/128))
- The `GenConverter` will unstructure iterables (`list[T]`, `tuple[T, ...]`, `set[T]`) using their type argument instead of the runtime class if its elements, if possible. These unstructuring operations are up to 40% faster.
([129](https://github.com/python-attrs/cattrs/issues/129))
- Flesh out `Converter` and `GenConverter` initializer type annotations.
([131](https://github.com/python-attrs/cattrs/issues/131))
- Add support for `typing.Annotated` on Python 3.9+. _cattrs_ will use the first annotation present. _cattrs_ specific annotations may be added in the future.
([127](https://github.com/python-attrs/cattrs/issues/127))
- Add support for dataclasses.
([43](https://github.com/python-attrs/cattrs/issues/43))