Remove support for Pyton 3.8 and older

Python 3.6 and 3.7 are not supported any of active stable branches now,
so no longer need to be supported.

Also remove Python 3.8 support according to [1].

[1]
https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/thread/FOWV4UQZTH4DPDA67QDEROAESYU5Z3LE/

Also add a few new versions currently tested.

Change-Id: Ie2617cbd48d6ec1ca40614784f944a5d29cd3b57

releasenotes/notes/drop-python38-support-59b0b2f406651efb.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Support for Python 3.8 has been dropped. Latest release of
+    telemetry-tempest-plugin to support python 3.8 is 2.3.0.
+    The minimum version of Python now supported is Python 3.9.

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://www.openstack.org/
-python_requires = >=3.6
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,10 +17,10 @@ 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
 
 [files]
 packages =