Remove python 3.6/3.7 support

... because python 3.6 and 3.7 are both EOL[1].

[1] https://devguide.python.org/versions/

Change-Id: I88d291746012c8bbfdff75f9cf0d2b1f5d397bf3

.zuul.yaml

@@ -3,9 +3,3 @@
       - openstack-python3-jobs
       - openstack-cover-jobs
       - publish-openstack-docs-pti
-    check:
-      jobs:
-        - openstack-tox-py36
-    gate:
-      jobs:
-        - openstack-tox-py36

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/reno/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,8 +15,6 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10