Pin Bandit on py2

See also: https://github.com/PyCQA/bandit/issues/654

Change-Id: If7f3ad8b275271d748426133232ed06c2a1cd1de

.zuul.yaml

@@ -542,7 +542,6 @@
               - ^(api-ref|doc|releasenotes)/.*$
               - ^test/probe/.*$
               - ^(.gitreview|.mailmap|AUTHORS|CHANGELOG|.*\.rst)$
-            # 2.25.0 had a test issue; see https://review.opendev.org/#/c/721518/
             voting: false
         - tempest-integrated-object-storage:
             irrelevant-files:

py2-constraints.txt

@@ -70,3 +70,6 @@ greenlet===0.4.15
 oslo.utils===3.42.1
 gitdb===0.6.4
 gitdb2===2.0.6
+
+# Projects that are known to have had a final py2-supporting release
+bandit===1.6.2