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: I8dca3772b3b5295b6216830289aa6e61d213d45c

openstack/mistral-extra,openstack/mistral-extra