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: I91baa08f496cb62df95381cc66274e0c2a9cdefb

releasenotes/notes/remove-py-36-37-5d61c3dcf682e0df.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

@@ -6,6 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/manila/latest/
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -14,10 +15,11 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
-    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 =