StartResponse callbacks are required to accept and optional third argument.
Currently, there is no good way to describe this using type hints.
Previously, a Union was used, but that causes mypy to complain about any call
of start_response().
* Use typing.Text
* Add InputStream and ErrorStream WSGI protocols
* Fix return type of WSGIApplication
* Add type hints to wsgiref.validate
* Replace _Bytes by plain bytes
* Add wsgiref.util
* Add wsgiref.headers
* ErrorWrapper.writelines() takes an Iterable
* Fix start_response return type
* Make Python 2 and 3 branches resemble each other more closely
* Use typing.NoReturn
* Fix WriteWrapper.writer type
* Change return type of WriteWrapper.writer to Any
