selftest: enable py3 for samba.tests.common

selftest: enable py3 for samba.tests.common

fix dsdb_Dn comparison for Python 3

In Python 3, the builtin `cmp` funtion was dropped. And the `__cmp__` magic
method in object is no longer honored, which is replaced by 6 new methods:
__eq__, __ne__, __lt__, __le__, __gt__, __ge__.

This caused `tests.CommonTests` failed with `py3_compatiable=True`.
Fixed by adding the above methods.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>