stack/slapos.cfg: pin slapos.toolbox to 0.128 on python2

slapos.toolbox no longer supports python2, this pins to the last version
where the test passed on python2.

In slapos.toolbox!116 (comment 181641)
we agreed that if we need to fix a python2 bug or backport something
critical, we can make a version 0.128.1 (based on 0.128 with the fix
for python2)

stack/slapos.cfg

@@ -360,6 +360,7 @@ pyparsing = 2.2.0
 pyrsistent = 0.16.1
 requests = 2.27.1
 selectors34 = 1.2
+slapos.toolbox = 0.128
 smmap = 0.9.0
 smmap2 = 2.0.5
 traitlets = 4.3.3