Explicitly depend on six in requirements file
Explicitly depend on six in requirements file

Six is used directly in oslo.db (and is not just a transitive
dependency) so it should be listed in the requirements file
just like other non-transitive dependencies.

Change-Id: I81d2d7d0db9142bace7ecf8882efbbcd605fb527

openstack/oslo.db,magic0704/oslo.db,JioCloud/oslo.db,akash1808/oslo.db,openstack/oslo.db,varunarya10/oslo.db