[requirements][all] We are now branched for stable/yoga

As the title states, openstack/requirements just branched. This means that master can start moving forward with upper-constraints and global-requirements changes.

1. If you have not branched yet you may start seeing dependency changes for the 'Z' release in your development branch (Yoga if you've not branched it yet). 2. If you have changes going into requirements' devepment branch but want to back port to yoga, remember that you'll have to create a backport patch for yoga (and other previous branches).