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

* [ ] 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.