TypedDict syntax variant with keyword arguments

[JukkaL]

Support defining typed dicts like this:

Point = TypedDict('Point', x=int, y=int)

[davidfstr]

FYI: I've started work on this.

[davidfstr]

FYI: I've started work on this.

Needless to say life has gotten busy on my end and I'm not expecting to resume work on this task in a reasonable timeframe.

I do have a partial patch that I started several months ago, but it may not be so useful since it is based on a very old version of the tree.

[JukkaL]

It's less clear if this is a good idea now that we also have the convenient class-based syntax, and we are going to use a keyword argument to specify that a typed dict may have missing items (see discussion at #2632).

[ilevkivskyi]

Some people might prefer the functional form. Both forms have their pluses and minuses, see the recent discussion in python/typing#427 and subsequent idea #3520

@gvanrossum What do you think?

[gvanrossum]

The functional form works in all Python versions.

[JukkaL]

This doesn't seem feasible now that we have the total=<bool> keyword argument to TypedDict. Also, TypedDict is officially supported in Python and we want to avoid mypy-specific extensions.