Implement StartResponse using a protocol

[srittau]

Also export ExcInfo and OptExcInfo from sys

Test case:

from sys import OptExcInfo
from typing import List, Tuple, Callable, Any
from wsgiref.types import StartResponse

def my_sr(status: str, headers: List[Tuple[str, str]], exc_info: OptExcInfo = None) -> Callable[[bytes], Any]:
    return lambda _: None

def wrong_sr1(status: str, headers: List[Tuple[str, str]]) -> Callable[[bytes], Any]:
    return lambda _: None

def wrong_sr2(status: str, headers: List[Tuple[str, str]], exc_info: OptExcInfo) -> Callable[[bytes], Any]:
    return lambda _: None

def cb(sr: StartResponse) -> None:
    pass

cb(my_sr)  # no error
cb(wrong_sr1)  # type error
cb(wrong_sr2)  # type error

[JelleZijlstra]

Looks good, but I'd prefer to keep the stub-only aliases prefixed with an underscore. You can still use them in other stubs.

[JelleZijlstra]

Then we should prefix these with an underscore.