Dataclasses-json

What's Changed
* Annotations test: Add `clean_exit=True` for `mypy_main()` by USSX-Hares in https://github.com/lidatong/dataclasses-json/pull/345
* Explicitly allow dataclasses_json.cfg.config() argument letter_case to accept LetterCase members by USSX-Hares in https://github.com/lidatong/dataclasses-json/pull/343


**Full Changelog**: https://github.com/lidatong/dataclasses-json/compare/v0.5.6...v0.5.7

* Override encoding before _asdict (312)
* Run tests using Python 3.9 and 3.10-alpha too (274)
* Skip newtype tests in 3.10

TODO
PRs merged Aug 25

Fix tuple keys in `dict` (297). Thanks pearceeverydaylabs

This release updates `pytest` to 6.5.2, because of a security fix in its transitive dependency `py` (which will be indirectly upgraded to `>=1.10`.

Note `pytest` is not installed by default with this package -- only when you specify want `dev` mode (see `setup.py` file).