Recent work on autodoc-utils added an import for typing-extensions,

but this wasn't added to the list of dependencies. That ended up

breaking builds.
This change adds that dependency, using 4.6 as the minimum version. This

is pretty old, but is the first version to introduce the

__orig_bases__ feature we're using. Most installs will have a newer

version, but this keeps the constraints from being too tight and causing

upgrades unnecessarily.

Successfully tested in a new virtualenv.