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/2and3/wsgiref/types.pyi

@@ -15,10 +15,12 @@
 # you need to use 'WSGIApplication' and not simply WSGIApplication when type
 # hinting your code.  Otherwise Python will raise NameErrors.
 
-from typing import Callable, Dict, Iterable, List, Any, Text, Protocol
+from sys import _OptExcInfo
+from typing import Callable, Dict, Iterable, List, Any, Text, Protocol, Tuple, Optional
+
+class StartResponse(Protocol):
+    def __call__(self, status: str, headers: List[Tuple[str, str]], exc_info: Optional[_OptExcInfo] = ...) -> Callable[[bytes], Any]: ...
 
-# (status: str, headers: List[Tuple[str, str]], exc_info = None)
-StartResponse = Callable[..., Callable[[bytes], Any]]
 WSGIEnvironment = Dict[Text, Any]
 WSGIApplication = Callable[[WSGIEnvironment, StartResponse], Iterable[bytes]]