Restricted: Allow datetime module class attributes.
    
To allow the class attributes, we add a helper method ``allow_class_attribute''.
 Why addng this is because allow_module can allow instance methods but not class
 attributes.
 For example, datetime.datetime.now(), datetime.datetime.max, dict.fromkeys(),
 Decimal.from_float(), epoll.fromfd(), itertools.chain.from_iterable() are class
 attributes in Python 2.7.
    
 Also, add a special in-advance-import for datetime.datetime.strptime().
 It is because datetime.datetime.strptime() imports _strptime by C function
 PyImport_Import which calls
 __import__(name, globals, locals, fromlist=['__doc__'], level=0).
 The "level=0" is not supported by AccessControl in Zope2. At the same time,
 the dummy from '__doc__'  is neither allowed in it by default.
 Therefore we import _strptime in advance in this monkey patch file.
 This prevents both importing _strptime with level=0, and accessing __doc__,
 when calling datetime.datetime.strptime() in Restricted environment.

/cc @vpelletier @jm @klaus

/reviewed-on nexedi/erp5!275