New RedirectionIterator and session send methods:

 + `send_single` replaces `send` with `allow_redirects=False`.
   That mode of `send` is deprecated, but preserved for backward
   compatibility.
 + `send_iter_redirects` replaces `resolve_redirects` (which is
   now deprecated, but, again, preserved for compatibility).
   It produces an iterator over all responses in an HTTP redirection
   chain, eliminating the need to special-case the first response.