Dropped the python 3.6/3.7 in zed cycle testing runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support.

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

Change-Id: Ic8cf4bf0ba12043d15e1ff31832f42cc14b3e021

setup.cfg

@@ -6,15 +6,13 @@ author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 license = Apache-2
 home_page = https://docs.openstack.org/freezer/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Programming Language :: Python
     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
     Development Status :: 5 - Production/Stable

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py39,py38,py36,pep8,pylint,docs
+envlist = py39,py38,pep8,pylint,docs
 skipsdist = True
 
 [testenv]
@@ -38,9 +38,6 @@ whitelist_externals =
 python_files = test_*.py
 norecursedirs  = .tox .venv 
 
-[testenv:py36]
-basepython = python3.6
-
 [testenv:py38]
 basepython = python3.8