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. Add release notes and update the python
classifier for the same.

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

Co-Authored-By: Ghanshyam Mann <gmann@ghanshyammann.com>
Change-Id: I998b34f884a2869d46ff05a179708a7f775e1615

.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-c3d8c440800ed885.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
 

tox.ini

@@ -39,16 +39,6 @@ passenv =
 commands =
   stestr --test-path=./placement/tests/functional run {posargs}
 
-[testenv:functional-py36]
-envdir = {toxworkdir}/py36
-commands =
-  {[testenv:functional]commands}
-
-[testenv:functional-py37]
-envdir = {toxworkdir}/py37
-commands =
-  {[testenv:functional]commands}
-
 [testenv:functional-py38]
 envdir = {toxworkdir}/py38
 commands =