erp5_mrp: add effective_time variable in production_packing_list_workflow

bt5/erp5_trade/WorkflowTemplateItem/portal_workflow/packing_list_workflow/variable_effective_time.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="Workflow Variable" module="erp5.portal_type"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>automatic_update</string> </key>
+            <value> <int>1</int> </value>
+        </item>
+        <item>
+            <key> <string>description</string> </key>
+            <value>
+              <none/>
+            </value>
+        </item>
+        <item>
+            <key> <string>for_catalog</string> </key>
+            <value> <int>1</int> </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>variable_effective_time</string> </value>
+        </item>
+        <item>
+            <key> <string>portal_type</string> </key>
+            <value> <string>Workflow Variable</string> </value>
+        </item>
+        <item>
+            <key> <string>status_included</string> </key>
+            <value> <int>1</int> </value>
+        </item>
+        <item>
+            <key> <string>title</string> </key>
+            <value>
+              <none/>
+            </value>
+        </item>
+        <item>
+            <key> <string>variable_default_expression</string> </key>
+            <value> <string>python:state_change.kwargs.get(\'effective_time\',\'\')</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>