[PR PATCH] selftest/common: fix dsdb_Dn comparison for Python 3

[github at samba.org]

There is a new pull request by catalyst-joe-guo against master on the Samba Samba Github repository

https://github.com/catalyst-joe-guo/samba py3-common
https://github.com/samba-team/samba/pull/151

selftest/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 at catalyst.net.nz>

A patch file from https://github.com/samba-team/samba/pull/151.patch is attached
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