Support dict with combination of positional arguments, keyword arguments and **args

[JukkaL]

This is a follow-up to #239.

[gvanrossum]

Is this about the following?

    a = dict(x=1, y=2)
    b = dict(a, z=3)

The first works but the second gets:

...: error: No overload variant of "dict" matches argument types [Any, builtins.int]

I have three live occurrences of this.

[JukkaL]

Yeah, that as well. I was thinking about dict(x=1, **d).

[gvanrossum]

Oh, the latter is probably less common?

[drewhaven]

Our code has a bunch of instances of the b = dict(a, z=3) kind of constructor, using it to copy dicts with some specific values changed.

[JukkaL]

The discussion at #1370 (comment) is relevant.