Update python testing as per zed cycle testing 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: I9c807c2bad39fed288314b1fb291d2a286aa09c3

.zuul.yaml

@@ -15,8 +15,6 @@
       - release-notes-jobs-python3
     check:
       jobs:
-        - openstack-tox-functional-py36
-        - openstack-tox-functional-py37
         - openstack-tox-functional-py38
         - openstack-tox-functional-py39
         - openstack-tox-pep8
@@ -48,8 +46,6 @@
             irrelevant-files: *gate-irrelevant-files
     gate:
       jobs:
-        - openstack-tox-functional-py36
-        - openstack-tox-functional-py37
         - openstack-tox-functional-py38
         - openstack-tox-functional-py39
         - openstack-tox-pep8

releasenotes/notes/drop-python-3-6-and-3-7-9db9b12a73106e26.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

@@ -12,7 +12,7 @@ project_urls =
     API Reference = https://docs.openstack.org/api-ref/placement/
     Source Code = https://opendev.org/openstack/placement
     Release Notes = https://docs.openstack.org/releasenotes/placement/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -24,8 +24,6 @@ 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
     Programming Language :: Python :: 3.9