python3: use six.moves for httplib imports

This adds six to the requirements.txt file in order to make
some HTTP-related imports work across Python 2's httplib and Python 3's
http.client modules. Tests were updated, including one change to the
location of HTTPConnection - moving it from being accessed where it was
imported rather than its canonical location inside of
six.moves.http_client.

Change-Id: Ibc4932b37dfdf195cd5091066914513af1876955
Signed-off-by: Chuck Short <chuck.short@canonical.com>