Update python testing as per zed cycle testing runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Add release notes and update the python
classifier for the same.

[1] https://governance.openstack.org/tc/reference/runtimes/zed.htm

Change-Id: I4e27be997dc35592813d89a211bce553cd7252d3

.zuul.yaml

@@ -91,7 +91,7 @@
 
 - project:
     templates:
-      - openstack-python3-xena-jobs
+      - openstack-python3-zed-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - release-notes-jobs-python3

releasenotes/notes/drop-python-3-6-and-3-7-e629c26cc46a52fe.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.6 & 3.7 support has been dropped. The minimum version of Python now
+    supported is Python 3.8.

setup.cfg

@@ -5,12 +5,12 @@ description_file =
     README.rst
 license = Apache License, Version 2.0
 home_page = https://docs.openstack.org/mistral/latest/
+python_requires = >=3.8
 classifiers =
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Environment :: OpenStack
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators

test-requirements.txt

@@ -14,4 +14,4 @@ tempest>=21.0.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
 testtools>=2.2.0 # MIT
 unittest2>=1.1.0 # BSD
-
+WebTest>= 3.0.0 # Apache-2.0