sqltest: decode strings (to the specified encoding ot 'utf-8') only in Python3

as it is done in other places of this file

product/ERP5Type/patches/sqltest.py

@@ -174,13 +174,13 @@ else: # For easy diff with original (ZSQLMethods 3.14)
             else:
                 if not isinstance(v, StringTypes):
                     v = str(v)
-                if isinstance(v, six.binary_type):
-                    v = v.decode('utf-8')
+                if six.PY3 and isinstance(v, six.binary_type):
+                    v = v.decode(self.encoding or 'UTF-8')
                 # The call to sql_quote__ can return something that is not
                 # a native string anymore!
                 v = md.getitem('sql_quote__', 0)(v)
-                if isinstance(v, six.binary_type):
-                    v = v.decode('utf-8')
+                if six.PY3 and isinstance(v, six.binary_type):
+                    v = v.decode(self.encoding or 'UTF-8')
                 # if v.find("\'") >= 0: v="''".(v.split("\'"))
                 # v="'%s'" % v
             vs.append(v)