Switch python3 versions of test jobs to match Train PTI

In Train, we will use python 3.6 and 3.7 for python3 runtime
in our gate jobs [1]. This commit also adds python 3.7.

[1] https://governance.openstack.org/tc/reference/runtimes/train.html

Change-Id: I8acde313e7e295cdecaa593dbeb41e8ee49afed5

zuul.d/projects.yaml

@@ -7,8 +7,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
-      - openstack-python37-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check: