Consider publishing release candidates to PyPI

[rsokl]

It could be helpful for typing-extensions to begin publishing nightlies or release-candidates to PyPI, so that projects that run CI against pre-releases can potentially catch bugs in typing-extensions before they get released.

E.g. hydra-zen's CI would have caught #181 before 4.6.0 got released.

[AlexWaygood]

Duplicate of #185