py2/py3: fix fromlist for __import__().

With fromlist='erp5.component.extention' (that should be ['erp5.component.extention']), it works in Python 2.
But in Python 3, we have an exception like:
  ModuleNotFoundError: No module named 'erp5.component.extension.erp5_version.InventoryBrain.'
With just fromlist=[''], like we do in XMLSyncUtils, it works on both Python 2 and Python 3.

product/ERP5Type/patches/DA.py

@@ -279,7 +279,7 @@ def getObjectMeta(original_function):
     # imported, which is enough to use a ZODB Extension as a brain.
     try:
       m = __import__('erp5.component.extension.%s' % module, globals(),
-                     {}, 'erp5.component.extension')
+                     {}, [''])
 
       o = getattr(m, name, None)
       if o is None: