tests: target DateTime 3 and use mock to monkey patch

mock has several advantages, the main one here is that it errors when
patching does not replace an existing attribute, which happens when we
are not patching the right place.

These attributes are not the same place in DateTime 2 and 3, this code
when using DateTime 2 was replacing the attributes, but in DateTime 3 it
was just creating new attributes that were never used. Update the code
to patch the DateTime 3 location

bt5/erp5_open_trade/TestTemplateItem/portal_components/test.erp5.testOpenOrder.py

@@ -26,17 +26,13 @@
 ##############################################################################
 import unittest
 import os
+import sys
 import time
+import mock
 from unittest import expectedFailure
 from Products.ERP5Type.tests.ERP5TypeTestCase import ERP5TypeTestCase
 from DateTime import DateTime
 
-# explicitly set Europe/Paris timezone
-os.environ['TZ']='Europe/Paris'
-time.tzset()
-DateTime._localzone0 = 'GMT+1'
-DateTime._localzone1 = 'GMT+2'
-DateTime._multipleZones = True
 
 class TestOpenOrder(ERP5TypeTestCase):
   """
@@ -47,6 +43,19 @@ class TestOpenOrder(ERP5TypeTestCase):
     return 'Test Open Order'
 
   def afterSetUp(self):
+    # explicitly set Europe/Paris timezone
+    # We use mock, to make sure we patch in the right place, but the stopping
+    # the patcher does not really work as we also have to set TZ
+    os.environ['TZ'] = 'Europe/Paris'
+    time.tzset()
+    for patcher in (
+      mock.patch.object(sys.modules['DateTime.DateTime'], '_localzone0', new='GMT+1'),
+      mock.patch.object(sys.modules['DateTime.DateTime'], '_localzone1', new='GMT+2'),
+      mock.patch.object(sys.modules['DateTime.DateTime'], '_multipleZones', new=True),
+    ):
+      patcher.start()
+      self.addCleanup(patcher.stop)
+
     if getattr(self.portal, '_run_after_setup', None) is not None:
       return