Implement StartResponse using a protocol (#2392)

* Add ExcInfo and OptExcInfo type aliases

* Implement StartResponse using a protocol

* Mark stub-only types with an underscore

* Remove wrong TODO note

python/mypy#1178 is about variable-length tuples, while exc_info()
always returns a tuple with length 3. Ideally, exc_info() would
return Union[Tuple[Type[_E], _E, TracebackType], Tuple[None, None, None]],
but that is a different issue.

stdlib/3/sys.pyi

@@ -14,6 +14,10 @@ from importlib.abc import MetaPathFinder
 
 _T = TypeVar('_T')
 
+# The following type alias are stub-only and do not exist during runtime
+_ExcInfo = Tuple[Type[BaseException], BaseException, TracebackType]
+_OptExcInfo = Tuple[Optional[Type[BaseException]], Optional[BaseException], Optional[TracebackType]]
+
 # ----- sys variables -----
 abiflags: str
 argv: List[str]
@@ -128,9 +132,7 @@ def _current_frames() -> Dict[int, Any]: ...
 def displayhook(value: Optional[int]) -> None: ...
 def excepthook(type_: Type[BaseException], value: BaseException,
                traceback: TracebackType) -> None: ...
-def exc_info() -> Tuple[Optional[Type[BaseException]],
-                        Optional[BaseException],
-                        Optional[TracebackType]]: ...
+def exc_info() -> _OptExcInfo: ...
 # sys.exit() accepts an optional argument of anything printable
 def exit(arg: object = ...) -> NoReturn:
     raise SystemExit()