Re-add python 3.6/3.7 in classifier

We have updated the yoga testing runtime to keep the
py36 testing.

- https://review.opendev.org/c/openstack/governance/+/820195

Unit tests job template is also updated to keep python
3.6 as a voting job. So with the py3.6 and py3.9 testing as voting
job template, we are keeping python 3.6, 3.7, 3.8, and 3.8 as
tested versions in the Yoga cycle.

- https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/820286

This commit re-add the python 3.6/3.7 versions in setup.cfg classifier.

Change-Id: Iaf4b4a82b5a50b3ef2dc793c50ef1b0981f37856

Author: Ghanshyam Mann

setup.cfg

@@ -17,6 +17,8 @@ classifier =
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9