fix type for itertools.product (#2129)

Fixes #1850.

The fix was already applied to Python 2, but the typevar-based solution there
leads to "cannot infer value of type variable" in mypy. I used the following
script to check:

```python
from itertools import product

reveal_type(product([1]))
reveal_type(product([1], ['x'], [False], [3.0], [(1,)], [('x',)], [{1}], [{1: 2}], repeat=5))
```

Author: JelleZijlstra

stdlib/2/itertools.pyi

@@ -153,9 +153,10 @@ def product(iter1: Iterable[Any],
             iter4: Iterable[Any],
             iter5: Iterable[Any],
             iter6: Iterable[Any],
-            iter7: Iterable[Any], *iterables: Iterable) -> Iterator[Tuple]: ...
+            iter7: Iterable[Any],
+            *iterables: Iterable[Any]) -> Iterator[Tuple[Any, ...]]: ...
 @overload
-def product(*iter: Iterable[_T], repeat: int) -> Iterator[Tuple[_T, ...]]: ...
+def product(*iterables: Iterable[Any], repeat: int) -> Iterator[Tuple[Any, ...]]: ...
 
 def permutations(iterable: Iterable[_T],
                  r: int = ...) -> Iterator[Sequence[_T]]: ...

stdlib/3/itertools.pyi

@@ -60,47 +60,43 @@ _T5 = TypeVar('_T5')
 _T6 = TypeVar('_T6')
 
 @overload
-def product(iter1: Iterable[_T1], *,
-            repeat: int = ...) -> Iterator[Tuple[_T1]]: ...
+def product(iter1: Iterable[_T1]) -> Iterator[Tuple[_T1]]: ...
 @overload
 def product(iter1: Iterable[_T1],
-            iter2: Iterable[_T2], *,
-            repeat: int = ...) -> Iterator[Tuple[_T1, _T2]]: ...
+            iter2: Iterable[_T2]) -> Iterator[Tuple[_T1, _T2]]: ...
 @overload
 def product(iter1: Iterable[_T1],
             iter2: Iterable[_T2],
-            iter3: Iterable[_T3], *,
-            repeat: int = ...) -> Iterator[Tuple[_T1, _T2, _T3]]: ...
+            iter3: Iterable[_T3]) -> Iterator[Tuple[_T1, _T2, _T3]]: ...
 @overload
 def product(iter1: Iterable[_T1],
             iter2: Iterable[_T2],
             iter3: Iterable[_T3],
-            iter4: Iterable[_T4], *,
-            repeat: int = ...) -> Iterator[Tuple[_T1, _T2, _T3, _T4]]: ...
+            iter4: Iterable[_T4]) -> Iterator[Tuple[_T1, _T2, _T3, _T4]]: ...
 @overload
 def product(iter1: Iterable[_T1],
             iter2: Iterable[_T2],
             iter3: Iterable[_T3],
             iter4: Iterable[_T4],
-            iter5: Iterable[_T5], *,
-            repeat: int = ...) -> Iterator[Tuple[_T1, _T2, _T3, _T4, _T5]]: ...
+            iter5: Iterable[_T5]) -> Iterator[Tuple[_T1, _T2, _T3, _T4, _T5]]: ...
 @overload
 def product(iter1: Iterable[_T1],
             iter2: Iterable[_T2],
             iter3: Iterable[_T3],
             iter4: Iterable[_T4],
             iter5: Iterable[_T5],
-            iter6: Iterable[_T6], *,
-            repeat: int = ...) -> Iterator[Tuple[_T1, _T2, _T3, _T4, _T5, _T6]]: ...
+            iter6: Iterable[_T6]) -> Iterator[Tuple[_T1, _T2, _T3, _T4, _T5, _T6]]: ...
 @overload
 def product(iter1: Iterable[Any],
             iter2: Iterable[Any],
             iter3: Iterable[Any],
             iter4: Iterable[Any],
             iter5: Iterable[Any],
             iter6: Iterable[Any],
-            iter7: Iterable[Any], *iterables: Iterable,
-            repeat: int = ...) -> Iterator[Tuple]: ...
+            iter7: Iterable[Any],
+            *iterables: Iterable[Any]) -> Iterator[Tuple[Any, ...]]: ...
+@overload
+def product(*iterables: Iterable[Any], repeat: int) -> Iterator[Tuple[Any, ...]]: ...
 
 def permutations(iterable: Iterable[_T],
                  r: Optional[int] = ...) -> Iterator[Tuple[_T, ...]]: ...