xreload likely breaks for classmethods

[d-maurer]

plone.reload.xreload._update_class assumes that Python 3 does not use method objects on class access. However, this is wrong for classmethods: accessing a @classmethod on a class returns a method object (not a function).

The _update_method function uses `six.get_unbound_function" in order to get a function. However, for Python 3, this is the identity function; thus, is does not change a method into a function object.

The upcoming dm.plonepatches.reload will use an auxiliary function def func(f): return getattr(f, "__func__", f) to transform a function or method object into the associated function. This works reliably for Python 2.7 and Python 3.