Change function.func_doc to function.__doc__
Change function.func_doc to function.__doc__

In python3 func_doc was renamed to __doc__, see [0].

[0]: https://docs.python.org/3.0/whatsnew/3.0.html#operators-and-special-methods
Closes-Bug: #1844660

Change-Id: I8585f8429acb7359e1f372a5e4f378ca5ce6b540

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl