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: I6042360da647b8378d515d5dad35a9077b77e46d

openstack/tosca-parser,openstack/tosca-parser