PyUnicode_Join(): Rewrote to use PySequence_Fast(). This doesn't do

PyUnicode_Join():  Rewrote to use PySequence_Fast().  This doesn't do
much to reduce the size of the code, but greatly improves its clarity.
It's also quicker in what's probably the most common case (the argument
iterable is a list).  Against it, if the iterable isn't a list or a tuple,
a temp tuple is materialized containing the entire input sequence, and
that's a bigger temp memory burden.  Yawn.