PyUnicode_Join(): Bozo Alert. While this is chugging along, it may

PyUnicode_Join():  Bozo Alert.  While this is chugging along, it may
need to convert str objects from the iterable to unicode.  So, if
someone set the system default encoding to something nasty enough,
the conversion process could mutate the input iterable as a side
effect, and PySequence_Fast doesn't hide that from us if the input was
a list.  IOW, can't assume the size of PySequence_Fast's result is
invariant across PyUnicode_FromObject() calls.