pwclient: Rework HTTP authentication

pwclient: Rework HTTP authentication

Transform the HTTP authentication class into a generic transport class.
The credentials become optional so this transport class is always used.

A side effect is to fix the Python 3 support for the authentication.
Fixes #59

It will help to bring proxy support while combining http/https and
authentication cases.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>
(cherry picked from commit 0bcb1bf6466461043bcb84c4856166105d5d5738)