[Python 3]basestring to six string_types

Summary:
Using for compatibility
from six import string_types
and
"string_types" for "basestring"
as it is still used in pykolab/utils.py

Reviewers: PyKolab, PyKolab Developers, sicherha

Reviewed By: PyKolab Developers, sicherha

Subscribers: sicherha

Differential Revision: https://git.kolab.org/D3403