[pkg] Use Twisted < 16.0.0
We have some test failures with 16.0.0, such as:
- test_put_content (unittest_server.ETwistHTTPTC)
- test_base_url (unittest_httptest.TwistedCWAnonTC)
(grafted from 10c12f31d0fe38c2e8814176fc42bf2ea210efdf)
sync_schema_props_perms(('State', 'state_of', 'Workflow'), commit=False)
sync_schema_props_perms(('State', 'name', 'String'))