Remove lxml from requirements

Module lxml seems to be unused in oslo, so we removed it from
requirements.txt

Change-Id: I68627f9d430f184a2daf4e8e4623d0d679ddbce1
excutils: replace unicode by six.u

This makes it more Python 3 compatible.

Change-Id: If640e551905836124657c79ab624e5717a0033a6
Merge "excutils: replace unicode by six.u"
Merge "Remove lxml from requirements"