Disallow using Undefined in arbitrary expressions

[JukkaL]

Using Undefined should only be valid as an initializer in an assignment statement (but not as a subexpression of an initializer). These should be disallowed:

if x is Undefined: ...
y = [Undefined] # type: ...
def f(a: int = Undefined) -> int: ...

These should still be fine:

x = Undefined # type: int
y = Undefined(int)

Not sure about this one:

a, b = Undefined, Undefined # type: (int, str)

Currently Undefined can be used as an expression.

See also python/typing#20 (comment).

[JukkaL]

As we are going to remove Undefined, this issue is no longer relevant.