Question

How should Astropy implement typing?
What is the timeline?
What Infrastructure needs to be built to enable this? e.g. mypy checker in test suite or submissions to python's type-shed.

OLD Description

See #10662

The python typing library is under active development and new and useful features are not available for py3.6-3.8. The typing_extensions library is now the official backport for all these types and is even developed within the main body of python (though offered as a separate install) — see https://github.com/python/typing/tree/master/typing_extensions.

As astropy moves to add static typing, the extended types options, in addition to the guaranteed stability and long-term support, would make this a useful and safe dependency.

I'm making this an issue, not PR, because I know how contentious adding new dependencies can be.

Edit: this would probably not be a permanent additional dependency since the features in typing_extension all become core python. When new features stop being added regularly and our python minveraion catches up, this can be dropped.

Edit: Now suggesting as an OPTIONAL dependency