s4/web_server: PY3: port to python3

s4/web_server: PY3: port to python3

Note: Unlike other libraries this library is been only built for the
configured python version. It depends on availability of 'swat' python
module.

The swat module is hosted externally, it seems not to have been modified
for a number of years, I don't think swat is python3 compatabile. These
changes are enough to get allow the samba binary to launch a web server
(which will just display a placeholder page announcing you need to install
swat). It maybe that removing this functionality is what we should do, but
that is a decision that can be made at a later time.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>