kubernetes/client/apis/admissionregistration_api.py line 120 uses async as a named parameter. But that's not allowed in python 3.7

This has been changed in the 8.0 branch but not in other branches.

We can't use the 8.0 branch because it's not compatible with K8s 1.10 - which is the most up to date stable K8s available on GCP.

Could the fix be ported to the 6.0 branch and a new release cut?