Use py36 job for safety checks

When using pipenv, the targeted version must be used when attempting to
do pipenv check. This change updates the safety dependency check to use
openstack-tox-py36  so the correct python version will be used.

Change-Id: I4b9f71cfaaa74724ecee7b40ea6e9362e836d261

.zuul.yaml

@@ -47,12 +47,13 @@
 
 - job:
     name: spyglass-dependency-vulnerability-check
-    parent: openstack-tox
+    parent: tox-py36
     voting: false
     timeout: 600
-    nodeset: spyglass-single-node
+    nodeset: ubuntu-bionic
     vars:
       tox_envlist: safety
+      bindep_profile: test py36
 
 - job:
     name: spyglass-docker-build-gate-ubuntu_xenial

tox.ini

@@ -48,7 +48,6 @@ deps =
   pipenv
   safety
 commands =
-  pipenv install --dev
   pipenv check
   safety check -r {toxinidir}/doc/requirements.txt --full-report