Drop helpers.next() and just use Python's built-in.

The python built-in 'next' function has an optional second parameter
that specifies what to return instead of excepting with StopIteration.
There is therefore no need for the function in helpers.py.

Signed-off-by: Aidan Hobson Sayers <aidanhs@cantab.net>
[rlb@defaultvalue.org: adjust commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
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>
Merge work to make helpers.next() match the newer Python next().