packaging: Properly flag universal wheel
packaging: Properly flag universal wheel

I'm not sure we've *ever* done this properly; see the PyPA docs:
https://packaging.python.org/guides/distributing-packages-using-setuptools/#universal-wheels

Change-Id: I8bb9e05f386076aa652b3955f0abf757d229afed

sohonetlabs/python-swiftclient,openstack/python-swiftclient,sohonetlabs/python-swiftclient,openstack/python-swiftclient