Make BufferReader.__next__() more compatible with builtin next()

[andeaseme]

The next() function should either raise StopIteration or yield the given default value instead of always returning None while the next value is unavailable.

https://docs.python.org/3/library/functions.html#next
next(iterator[, default])
Retrieve the next item from the iterator by calling its next() method. If default is given, it is returned if the iterator is exhausted, otherwise StopIteration is raised.

┆Issue is synchronized with this Asana task by Unito