python3: Fix import compatibility.

Python3 reorganized the standard library and moved several functions
to different modules. Six provides a consistent interface
to them through the fake six.moves module.

However, the urlparse, urllib2, etc modules have been combined
into one module which Six does not support so do it the old
fashioned way.

Change-Id: Idfb4f8eb538dcc98a1f7befc4487635d7f483973
Signed-off-by: Chuck Short <chuck.short@canonical.com>