Release 4.6.3 planning

[JelleZijlstra]

I'm about to release bugfix release 4.6.3 primarily for #208. However, I want to exclude #173 as that's arguably a new feature, so I'm releasing off the branch https://github.com/python/typing_extensions/tree/463. I'll upload the release momentarily but I'm opening this issue for information.

This should be the last bugfix release for 4.6; afterwards we can start merging more features in preparation for the next feature release.

[JelleZijlstra]

Done.