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: If88b377f25d9b3e64dc049ae219ed6d5f4782d43

setup.cfg

@@ -7,7 +7,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/python-heatclient/latest
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: Console
@@ -18,8 +18,6 @@ classifier =
     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
     Programming Language :: Python :: 3 :: Only