software/erp5-zope2: correctly compile in python2

This is a fixup of f5b80301.

software/erp5-zope2/software.cfg

 [buildout]
 extends =
   ../../stack/erp5-zope2/buildout.cfg
+  ../../stack/slapos-py2.cfg
 
 [erp5]
 branch = zope2