special-case overload for itertools.product with 'repeat' (#1410)

Add an additional overload for the `repeat = n` case. Since we don't know 
the numeric value of `repeat`, this just produces an iterator over an 
arbitrary-length tuple.

Author: matthiaskramm

stdlib/2/itertools.pyi

@@ -128,6 +128,8 @@ def product(iter1: Iterable[Any],
             iter5: Iterable[Any],
             iter6: Iterable[Any],
             iter7: Iterable[Any], *iterables: Iterable) -> Iterator[Tuple]: ...
+@overload
+def product(*iter: Iterable[_T], repeat: int) -> Iterator[Tuple[_T, ...]]: ...
 
 def permutations(iterable: Iterable[_T],
                  r: int = ...) -> Iterator[Sequence[_T]]: ...