Use only Yoga tests

TC has decided to keep support for Python 3.6 during the Yoga cycle.
For more info see [1]

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-December/026164.html

Change-Id: I19b1182cc17aab0700e4a8a982b7cb9e63a62ecb

.zuul.yaml

@@ -89,20 +89,10 @@
       - release-notes-jobs-python3
     check:
       jobs:
-        # NOTE(rpittau): yoga jobs support py38 and py39 but we want to keep
-        # testing py36 to maintain compatibility with older distributions like
-        # CentOS Stream 8, Ubuntu Bionic, Debian 10, and openSuse Leap
-        # Remove after Yoga
-        - openstack-tox-py36
         - ironic-prometheus-exporter-redfish
         - bifrost-integration-tinyipa-keystone-centos-8
     gate:
       queue: ironic
       jobs:
-        # NOTE(rpittau): yoga jobs support py38 and py39 but we want to keep
-        # testing py36 to maintain compatibility with older distributions like
-        # CentOS Stream 8, Ubuntu Bionic, Debian 10, and openSuse Leap
-        # Remove after Yoga
-        - openstack-tox-py36
         - ironic-prometheus-exporter-redfish
         - bifrost-integration-tinyipa-keystone-centos-8

setup.cfg

@@ -18,6 +18,8 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9