Mark string.join() as done. Turns out string_join() works "for free" now,

because PySequence_Fast() started working for free as soon as
PySequence_Tuple() learned how to work with iterators.  For some reason
unicode.join() still doesn't work, though.

Misc/NEWS

@@ -23,8 +23,9 @@ Core
     max()
     min()
     reduce()
-    XXX TODO string.join(), unicode.join()
+    string.join()
     tuple()
+    XXX TODO unicode.join()
     XXX TODO zip()
     XXX TODO 'x in y'