Remove redundant __nonzero__ methods
From https://docs.python.org/2/reference/datamodel.html#specialnames :
When this method is not defined, __len__() is called, if it is
defined, and the object is considered true if its result is nonzero.
Also helpful for an eventual migration to py3k, as __nonzero__ has been
replaced with __bool__, which also defaults to looking at __len__ if
undefined.