Updated openstack/openstack
Project: openstack/python-neutronclient  39b15671f8e33e3d9eafe73648ee771c7eedb1d5
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Add -U to pip install command in tox.ini

Ask pip to upgrade packages in the virtualenv so that a long-lived
virtualenv is still kept up to date as external dependencies change.

Change-Id: I0256cf4cc3aa7675956762dcf056e2709d36813a