2to3: Apply nonzero fixer.

In Python 3 the `__nonzero__` class method is replaced by `__bool__`.
This only affects the MaskedArray class in numpy/oldnumeric/ma.py file
and the simplest solution is to provide both methods. I have my doubts
that the fixed up Python 3 version was correct or even tested, but I
think the current solution should work for as long as oldnumeric stays
in numpy.

Closes #3073.