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: If6c82699a088644aa7515021f62b1d08cb6c61fb

setup.cfg

@@ -6,11 +6,10 @@ summary = The OpenStack Backup Restore and Disaster Recovery as a Service Platfo
 description_file = README.rst
 home_page = https://docs.openstack.org/freezer/latest/
 license = Apache-2
+python_requires = >=3.8
 classifier =
     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
     Development Status :: 5 - Production/Stable

tox.ini

@@ -54,9 +54,6 @@ basepython = python3.9
 [testenv:py38]
 basepython = python3.8
 
-[testenv:py36]
-basepython = python3.6
-
 [testenv:docs]
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}