complex should support SupportsComplex

[shoyer]

As @gvanrossum points out in python/mypy#4592:

Both the Py2 and Py3 definition of class complex there are lacking several things -- a base class of SupportsComplex and an overload of the constructor to take a SupportsComplex value.

[JukkaL]

Both the Py2 and Py3 definition of class complex there are lacking several things -- a base class of SupportsComplex and an overload of the constructor to take a SupportsComplex value.

Since SupportsComplex is a protocol, there's no need for an explicit SupportsComplex base class, similar to how int doesn't have a SupportsInt base class and so on.

[gvanrossum]

Good point, I always forget about Protocols. But the complex class definitely needs another constructor overload.