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.html

This also removes broken lower-constraints job.

Change-Id: I5c9f3c031d74e0b1db5b49c0634139f9b6f49b5e

.zuul.yaml

@@ -28,8 +28,7 @@
 - project:
     templates:
       - check-requirements
-      - openstack-lower-constraints-jobs
+      - openstack-python3-zed-jobs
-      - openstack-python3-ussuri-jobs
       - openstackclient-plugin-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/python-zaqarclient/latest/
-python-requires = >=3.6
+python-requires = >=3.8
 classifier =
     Development Status :: 4 - Beta
     Environment :: Console
@@ -20,8 +20,8 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.8
-    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.9
 
 [files]
 packages =