filter function: make the callable of the first overload non-optional so that mypy is able to select the second overload for the case. (#1855)

Author: gwk

stdlib/3/builtins.pyi

@@ -804,8 +804,7 @@ def eval(source: Union[str, bytes, CodeType], globals: Optional[Dict[str, Any]]
 def exec(object: Union[str, bytes, CodeType], globals: Optional[Dict[str, Any]] = ..., locals: Optional[Mapping[str, Any]] = ...) -> Any: ...
 def exit(code: Any = ...) -> NoReturn: ...
 @overload
-def filter(function: Optional[Callable[[_T], Any]],
-           iterable: Iterable[_T]) -> Iterator[_T]: ...
+def filter(function: Callable[[_T], Any], iterable: Iterable[_T]) -> Iterator[_T]: ...
 @overload
 def filter(function: None, iterable: Iterable[Optional[_T]]) -> Iterator[_T]: ...
 def format(o: object, format_spec: str = ...) -> str: ...