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: I57fbd274c237a9f48820a13b123df64c028d34f4

setup.cfg

@@ -6,7 +6,7 @@ summary = Freezer - Backup as a Service User Interface
 description_file = README.rst
 home_page = https://github.com/openstack/freezer-web-ui
 license = Apache-2
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -21,7 +21,6 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Topic :: Internet :: WWW/HTTP

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py38,py39,pep8,pylint,docs
+envlist = py38,py39,pep8,pylint,docs
 minversion = 2.0
 skipsdist = True
 
@@ -18,9 +18,6 @@ deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/up
 commands =
    python manage.py test {posargs}
 
-[testenv:py36]
-basepython = python3.6
-
 [testenv:py38]
 basepython = python3.8