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: Ib6c6e3ffae0a1310c111f2fadbf97a8e6ce0c67f

openstack/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard