Add `Color.copy()` and `Color.__copy__()` methods

[aatle]

Mutable collections almost always have a .copy() method.
It can be faster and more direct than using the (overloaded) constructor to copy.

__copy__ should also be added, for a more efficient copy.copy(). (note it is already supported via the copyreg registration used for color pickling.)

[ankith26]

+1: Vector and Rect already have copy so it makes sense to add it to Color as well