Bump requirement storpool's version, remove six

Six was transitively required by an old version of storpool. The new
version takes care of this internally, and six is not required.

Change-Id: I88bd3f79499fc0990913dc82ff3f56d44a5608fa
diff --git a/requirements.txt b/requirements.txt
index 08285c9..8d92e1a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,4 @@
 ops
 git+https://opendev.org/openstack/charm-ops-openstack#egg=ops_openstack
-six
-storpool
+storpool >= 7.2.0, < 8
 storpool.spopenstack