py3: Add helper functions to write code for both python2 and python3.

* Add functions to properly convert/handle bytes/str/unicode (from
  slapos.util) (stricter than six.ensure_{text,str}()).
* Python3 {keys,values,items}() now returns dict_{keys,values,items} rather than
  real list() so add ensure_list() to create a list() when it is latter modified
  but do nothing on Python2 for performance sake.

product/ERP5Type/Utils.py

@@ -554,6 +554,24 @@ def checkPythonSourceCode(source_code_str, portal_type=None):
 # Python 2-3 compat
 #####################################################
 
+if str is bytes:
+  bytes2str = str2bytes = lambda s: s
+  def unicode2str(s):
+    return s.encode('utf-8')
+else:
+  def bytes2str(s):
+    return s.decode()
+  def str2bytes(s):
+    return s.encode()
+  def unicode2str(s):
+    return s
+
+if six.PY3:
+  def ensure_list(o):
+    return list(o)
+else:
+  ensure_list = lambda x: x
+
 def with_metaclass(meta, *bases):
   """
   Function from jinja2/_compat.py. License: BSD (copy/paste here for

product/ERP5Type/__init__.py

@@ -180,7 +180,7 @@ ModuleSecurityInfo('Products.ERP5Type.Utils').declarePublic(
     'convertToMixedCase', 'cartesianProduct', 'sleep', 'getCommonTimeZoneList',
     'int2letter', 'getMessageIdWithContext', 'getTranslationStringWithContext',
     'Email_parseAddressHeader', 'guessEncodingFromText',
-    'isValidTALESExpression')
+    'isValidTALESExpression', 'ensure_list')
 
 allow_module('Products.ERP5Type.Message')
 ModuleSecurityInfo('Products.ERP5Type.Message').declarePublic('translateString')