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

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

Change-Id: I3892493c9a9add0b5d2e80e1f764e2eb37b047ea

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/ironic/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -16,8 +16,6 @@ classifier =
     Programming Language :: Python
     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