Add and use `TFL.Meta.BaM`: Python-3 compatibility

Specify `bases` and `metaclass` in syntax compatible to Python 2 & 3.

Use TFL.Meta.BaM in class statements like this ::

    class A_Class (BaM (A_Base_1, A_Base_2, metaclass = A_Metaclass)) :
        pass
Add `auth_required`, `cutoff_date`

* Consider those in hidden
Remove terminally broken module

Code like the following needs to be taken out and shot::

    def _subset_2 (self) :
        for i, j in enumerate (pyk.iterkeys (self._content)) :
            for k in self._content.keys () [i+1 :] :
                yield (j, k)
Improve Python-3 compatibility of package TFL

* Not yet the subpackages except TFL.Meta