The Python 3.6 and Python 3.7 Support has been dropped since zed

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

Change-Id: I2725aca3cd5620bd16d086298134f3735e7c338f
diff --git a/setup.cfg b/setup.cfg
index a8d2496..110ab8e 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -5,7 +5,7 @@
 author = OpenStack
 author_email = openstack-dev@lists.openstack.org
 url = http://www.openstack.org/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -14,8 +14,6 @@
     Operating System :: POSIX :: Linux
     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