Remove support for Pyton 3.6 and 3.7

Tempest and its plugins are branchless and need to support all active
stable branches. However none of the maintained branches currently
support these old python versions.

Also add a few new versions currently tested.

Change-Id: Ibf38ffbfddf01373b6c928c7a5841040ad85619c

releasenotes/notes/remove-py-36-37-d7724851f8db349d.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Support for Python 3.6 and 3.7 has been removed. Now the minimum version
+    supported is 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/designate-tempest-plugin/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Environment :: No Input/Output (Daemon)
@@ -18,10 +18,11 @@ 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
+    Programming Language :: Python :: 3.10
+    Programming Language :: Python :: 3.11
+    Programming Language :: Python :: 3.12
     Topic :: Internet :: Name Service (DNS)
 
 [files]