software/slapos-testing: Fix Python2-incompatible versions

statsmodels and websocket-client were pinned to Python3-only versions,
and slapos.toolbox was installed in develop mode from a Python3-only
revision.

software/slapos-testing/software-py2.cfg

@@ -9,5 +9,10 @@ extends =
 eggs +=
   ${re6stnet-setup:egg}
 
+[slapos.toolbox-setup]
+recipe = zc.recipe.egg
+eggs = ${:egg}
+
 [versions]
 pathlib = 1.0.1
+slapos.toolbox = 0.128.1

stack/slapos.cfg

@@ -409,9 +409,10 @@ selectors34 = 1.2
 slapos.toolbox = 0.128.1
 smmap = 0.9.0
 smmap2 = 2.0.5
-statsmodels = 0.11.1
+statsmodels = 0.11.0
 traitlets = 4.3.3
 uritemplate = 3.0.0
+websocket-client = 0.59.0
 Werkzeug = 1.0.1
 zipp = 1.2.0:whl