Deprecate our next methods, my_iterator.next() --> next(my_iterator)

This was based on running 2to3 -f next, and then manually
adding deprecated alias for the next method under Python 2
to preserve backward compatibility (except for SearchIO
where we are free to break the API like this).

Note this also disables the next fixer in do2to3.py
Update Tutorial, my_iterator.next() --> next(my_iterator)