2to3: Apply next fixer.

The next builtin has been available since Python 2.6 and allows
`it.next()` to be replaced by `next(it)`. In Python 3 the `next` method
is gone entirely, replaced entirely by the `__next__` method. The next
fixer changes all the `it.next()` calls to the new form and renames the
`next` methods to `__next__`. In order to keep  Numpy code backwards
compatible with Python 2, a `next` method was readded to all the Numpy
iterators after the fixer was run so they all contain both methods. The
presence of the appropriate method could have been made version
dependent, but that looked unduly complicated.

Closes #3072.