First attempt to fix 'Acquisition problem' when encountering cyclic hierarchies via __parent__ pointers. [hannosch, nouri]

Added a test to Acquisition that shows the current segmentation fault problem, when Acquisition goes in circles.

(Also some comment cosmetics in _Acquisition.c)

* no need to introduce another variable where we check for a __parent__ attribute
* clean up after failed getattr (it throws an AttributeError)
* properly DECREF the __parent__ attribute when it's no longer needed and
  the wrapper that is temporarily created from the __parent__ attribute.

style of the overall file

isn't acquisition wrapped.

(they point to the same thing).

in the original editing)