ERP5Type.Utils: returns an imutable copy of pytz.common_timezones

This also fixes the problem that in recent pytz common_timezones is a LazyList that does not work in restricted python

product/ERP5Type/Utils.py

@@ -1387,8 +1387,8 @@ def getCommonTimeZoneList():
   try:
     from pytz import common_timezones
   except ImportError:
-    return []
-  return common_timezones
+    return ()
+  return tuple(common_timezones)
 
 
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