[python] Convert unicode to six.u

[kenjones-cisco]

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Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

unicode does not exist in py3, therefore since six is already imported
switch to using six.u so that unicode is used on py2 and str is used
on py3.

@taxpon @frol @mbohlool @cbornet
CC @wing328

[frol]

LGTM