Docs: Update `TypedDict` import statements (#16958)
Since Python 3.8, `TypedDict` has been available from the `typing`
module.
As Python 3.8+ is needed to use mypy
(https://github.com/python/mypy/blob/master/setup.py#L12), then it's
best for the docs to reflect Python 3.8+ usage.
For previous versions, there's already a disclaimer on the page that
explains that `typing_extensions` must be used:
https://github.com/python/mypy/blob/master/docs/source/typed_dict.rst?plain=1#L102-L110
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
