python: Make top-level samba modules Python 3 compatible

python: Make top-level samba modules Python 3 compatible

New file compat.py will help with porting to Python 3. For now, it
contains only PY3 variable based on six.PY3 which simplifies
condition mentioned below.

The added `if not PY3` conditions enable us to bootstrap running
tests with Python 3 even if most modules are not ported yet.
The plan is to move modules outside this condition as they are ported.
The `PY3` condition is currently used only in tests and for
the samba._ldb module which is not ported yet and has a lot of
dependencies.

The other changes are related to differences between Python 2 and 3.
Python 2.6 introduced the `0o` prefix for octal literals as an
alternative to plain `0`. In Python 3, support for plain `0` is
dropped and octal literals have to start with `0o` prefix.
Python 2.6 introduced a clearer `except` syntax:
`except ExceptionType as target:` instead of
`except ExceptionType, target:`. In Python 3, the old syntax
is no longer allowed.

Signed-off-by: Lumir Balhar <lbalhar@redhat.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>