Fix fo() backdoor command for non-class objects

The backdoor command fo() uses isinstance() to check if an object is an
instance of a class. This only works with objects that have a __class__
attribute, else an AttributeError is raised by isinstance(). This is
seldomly the case, though if there is one such object fo() will cease to
work. Therefore we need to protect us against this case by checking for
a __class__ attribute before calling isinstance().

An example for an object without __class__ would be
functools._lru_list_elem.

Change-Id: Ia4c5cbdc249535d36f6e71f7b2a7359bc6fdf219
Closes-Bug: #1946072