With the move to the Victoria job template in
https://review.opendev.org/#/c/722681/, the py37 jobs no
longer get run, so the check and gate job entries can
be removed.
Added a keepalived py38 KillFilter line to match the py36
and py37 ones.
Also updated TESTING.rst to use py38 in all examples.
Change-Id: Ief793b54d53c3239cfb24278e88e4f4189bbc2c2