Make 2-arg iter() retrun type match passed callable's return type (#3326)

This is a continuation of #3291, which was the initial fix for #3201.

The 2-arg version of iter() turns a callable into an iterator. The
changes made in #3291 introduce an Any return type for both the
callable's return type and the iterator's type, while in reality the
return type of the function is always the same as the iterator's type.

Author: wbolster

stdlib/2/__builtin__.pyi

@@ -1193,7 +1193,7 @@ def iter(__iterable: Iterable[_T]) -> Iterator[_T]: ...
 @overload
 def iter(__function: Callable[[], Optional[_T]], __sentinel: None) -> Iterator[_T]: ...
 @overload
-def iter(__function: Callable[[], Any], __sentinel: Any) -> Iterator[Any]: ...
+def iter(__function: Callable[[], _T], __sentinel: Any) -> Iterator[_T]: ...
 def isinstance(__o: object, __t: Union[type, Tuple[Union[type, Tuple[Any, ...]], ...]]) -> bool: ...
 def issubclass(__cls: type, __classinfo: Union[type, Tuple[Union[type, Tuple[Any, ...]], ...]]) -> bool: ...
 def len(__o: Sized) -> int: ...

stdlib/2and3/builtins.pyi

@@ -1193,7 +1193,7 @@ def iter(__iterable: Iterable[_T]) -> Iterator[_T]: ...
 @overload
 def iter(__function: Callable[[], Optional[_T]], __sentinel: None) -> Iterator[_T]: ...
 @overload
-def iter(__function: Callable[[], Any], __sentinel: Any) -> Iterator[Any]: ...
+def iter(__function: Callable[[], _T], __sentinel: Any) -> Iterator[_T]: ...
 def isinstance(__o: object, __t: Union[type, Tuple[Union[type, Tuple[Any, ...]], ...]]) -> bool: ...
 def issubclass(__cls: type, __classinfo: Union[type, Tuple[Union[type, Tuple[Any, ...]], ...]]) -> bool: ...
 def len(__o: Sized) -> int: ...