Make a top-level TypedDict page

[hauntsaninja]

This just moves content around (with minimal editing to make the moves make sense).

TypedDict has been the target for several features, including some that are not yet documented. There was another PEP drafted today that was TypedDict themed. It's also pretty popular with users.

Linking #13681

[tmke8]

Maybe the class-based syntax should be introduced before the functional syntax? It seems to me the class-based syntax is nowadays preferred. I only see the functional syntax recommended when a key is not a valid variable name (e.g., it contains a hyphen or it coincides with a reserved keyword).

[JelleZijlstra]

I agree with the feedback that we should focus on the class-based syntax, and mention the functional syntax only as an alternative for rare cases. However, that might be better done in a separate PR, leaving this one focused on the move.

Other feedback:

• Consider using :py:data: to link to the stdlib docs, like we do for NewType.
• Do we have a general stance on when to use typing_extensions imports in the docs? TypedDict has been in the stdlib since 3.8.

[hauntsaninja]

Thanks, will make actual edits in a separate PR.

Do we have a general stance on when to use typing_extensions

I don't think we do. My views:

• 3.7 is close enough to EOL that I don't mind having random snippets in examples assume 3.8. I'm doing a big docs editing pass now, I may as well save us some time in four months (we may still want to support 3.7 for a little longer, but we don't need to target our educational material at those users)
• We should still have typing_extensions documented in places that it's helpful, it just doesn't necessarily need to be every snippet
• We should also improve the error message mypy gives when imports are missing from typing

[hauntsaninja]

PR for the suggestion here: #14591