drop mock from lower-constraints
drop mock from lower-constraints

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we don't need it
in lower-constraints.

These changes will help us to drop `mock` from openstack/requirements

Change-Id: I54a2fde199c151eee0ca81c0db840609088803ac

openstack/os-win