Only define helpers.next() if Python's isn't new enough.

Only define helpers.next() if Python's isn't new enough.

Since helpers.next() now matches the Python built-in's semantics
(since Python 2.6), just rely on the built-in when it's new enough.
Otherwise, set up a matching replacement.

Borrow the fallback next documentation string from Python.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>