Support declaring TypedDict type with class-based syntax (for Python 3.6+)

[davidfstr]

• Support basic declaration:

class Point2D(TypedDict):
    x: int
    y: int

• Support class-based inheritance as syntactic sugar for copying fields

class Point3D(Point2D):
    z: int

There is already support for the above syntaxes at runtime but the typechecker doesn't reconcognize it yet.

[ilevkivskyi]

@davidfstr I wanted to work on this soon. Do you prefer to do it yourself?

[davidfstr]

@ilevkivskyi Go for it. Help is definitely welcome, as my coding time outside work is highly constrained.

In case it's of interest the next item I'm working on is #2492 and #2739 (together).

[ilevkivskyi]

@davidfstr Here is the PR #2808 I promised.

[jonmcoe]

is it possible to inherit in python2?

I see error: TypedDict class syntax is only supported in Python 3.6 when trying to inherit in the class fashion above. wondering if there is another approach