The requirements job was failing since it doesn't allow duplicate
requirements like we have in test-requirements.txt and
test-requirements-py3.txt. test-requirements-py3.txt can now be
removed since python-memcached supports python3.
Also, updated the requirements using the update-requirements
script from commit ae4aa204367b97170a808ad6ef7261959c7efae1 .
Change-Id: Icfc7cedd3fe7ac48bb04f9acbb73d0b8d0190721