* Use HTTPMessage for the headers parameter of HTTP event handlers
While the documentation of `BaseHandler.http_error_default()` describes
the `hdrs` (`headers` in most other handlers) as "a mapping object with
the headers of the error", the implementation that is located in
`URLopener._open_generic_http()` will pass `response.msg` instead,
which is of type `http.client.HTTPMessage`.
* Use Message for the headers parameter of HTTPError
When the standard library constructs `HTTPError`, it will
pass an `http.client.HTTPMessage`, which is a subclass of
`email.message.Message`. Picking the superclass for the
annotations gives users the flexibility to for example
the result of the `email.message_from_X()` functions.
The only thing unique to `HTTPMessage` is the undocumented
`getallmatchingheaders()` method, which is only called by
`http.server.CGIHTTPRequestHandler.run_cgi()`. That class
gets its headers from `http.client.parse_headers()` and not
from `HTTPError`, so I think it's safe to use `Message`
as the annotation.
While these implementations don't matter for the 'typing' module
itself, these are also imported to serve as the implementations
for the 'collection.abc' module.
Fixes#3029
Maarten ter Huurne