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.html

Change-Id: I253757e2035dbe667905f011f9d05cc5d94e669e

releasenotes/notes/drop-python-3-6-and-3-7-3a90d172a5e43660.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

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/senlin/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -16,8 +16,6 @@ classifier =
     Programming Language :: Python
     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

@@ -44,8 +44,8 @@ whitelist_externals =
 basepython = python3
 commands = oslo_debug_helper -t senlin/tests/unit {posargs}
 
-[testenv:debug-py36]
-basepython = python3.6
+[testenv:debug-py38]
+basepython = python3.8
 commands = oslo_debug_helper -t senlin/tests/unit {posargs}
 
 [testenv:pep8]