Drop support for Python 3.6 and 3.7

It has been removed from Zed and we're now testing with Python 3.9 in
tinyipa and Python 3.8 in CentOS Stream 9

Change-Id: I028121d593b910e585f44e464c7fcb3e635420e8

setup.cfg

@@ -7,7 +7,7 @@ author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/ironic-python-agent/
 summary = Ironic Python Agent Ramdisk
 license = Apache-2
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: System Administrators
@@ -18,8 +18,6 @@ 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