pyldb: Add Python 3 compatibility (except strings)

pyldb: Add Python 3 compatibility (except strings)

- Use a macro for adding constants to module
(This also ensures that the Python constants have the same
name as the C ones. One existing misspelling is retained.)

- Use new module initialization for Python 3

- Use rich comparison for ldb objects

- Prepare tests for Python 3

Signed-off-by: Petr Viktorin <pviktori@redhat.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>