component/zodbtools: update py3 test to use ZODB6, repair py2-ZODB5

See merge request !1664

component/zodbtools/test-common.cfg

@@ -43,6 +43,8 @@ inline =
 
 [versions]
 freezegun = 1.5.1:whl
+ZopeUndo = 6.0
 
 [versions:python2]
 freezegun = 0.3.15
+ZopeUndo = 5.0

component/zodbtools/test-zodb5-py2.cfg

@@ -2,5 +2,8 @@
 
 [buildout]
 extends =
-  test-zodb5.cfg
+  test-common.cfg
   test-py2.cfg
+
+[ZODB]
+major = 5

component/zodbtools/test-zodb5.cfg → component/zodbtools/test-zodb6.cfg

-# SlapOS software release to test zodbtools/ZODB5-py3 on Nexedi testing infrastructure.
+# SlapOS software release to test zodbtools/ZODB6-py3 on Nexedi testing infrastructure.
 
 [buildout]
 extends = test-common.cfg
 
 [ZODB]
-major = 5
+major = 6