Update python testing as per zed cycle teting runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Removing the py36 centos8 job as well as
updating the python classifier also to reflect the same.

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

Change-Id: I269d1ca0b763d7806a3cec9d1be425d68f68fa67

releasenotes/notes/drop-python-3-6-and-3-7-77af6bd3473ea5ba.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.6 & 3.7 support has been dropped. The minimum version of Python now
+    supported is Python 3.8.

setup.cfg

@@ -22,7 +22,7 @@ license = Apache License, Version 2.0
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://www.openstack.org/software/releases/mitaka/components/murano
-python-requires = >=3.6
+python-requires = >=3.8
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -34,8 +34,6 @@ 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
 
 [files]

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py38,pep8
+envlist = py38,pep8
 minversion = 2.0
 skipsdist = True