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: I787414dcf25143c0ce2dc31a672d36e9fe75b746

openstack/solum,stackforge/solum,openstack/solum,stackforge/solum