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
diff --git a/releasenotes/notes/remove-py-36-37-d7724851f8db349d.yaml b/releasenotes/notes/remove-py-36-37-d7724851f8db349d.yaml
new file mode 100644
index 0000000..97958a9
--- /dev/null
+++ b/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 .