software/erp5-py3: use OPC UA POC branch

We also need various ERP5 side changes for the new OPC UA support
feature. During development we therefore use a specific ERP5 branch.
Once merged, the ERP5 side patches should also be merged and this
patch can be dropped.

software/erp5/software-py3.cfg

 [buildout]
 extends =
   ../../stack/erp5/buildout-py3.cfg
+
+[erp5]
+repository = http://lab.nexedi.com/levin.zimmermann/erp5
+branch = opcua-poc