Change list.extend() error msgs and NEWS to reflect that list.extend()

now takes any iterable argument, not only sequences.

NEEDS DOC CHANGES -- but I don't think we settled on a concise way to
say this stuff.

Misc/NEWS

@@ -100,7 +100,8 @@ Core
     map(), filter(), reduce(), zip()
     list(), tuple() (PySequence_Tuple() and PySequence_Fast() in C API)
     max(), min()
-    .join() method of strings
+    join() method of strings
+    extend() method of lists
     'x in y' and 'x not in y' (PySequence_Contains() in C API)
     operator.countOf() (PySequence_Count() in C API)
 

Objects/listobject.c

@@ -644,7 +644,7 @@ listextend_internal(PyListObject *self, PyObject *b)
 static PyObject *
 list_inplace_concat(PyListObject *self, PyObject *other)
 {
-	other = PySequence_Fast(other, "argument to += must be a sequence");
+	other = PySequence_Fast(other, "argument to += must be iterable");
 	if (!other)
 		return NULL;
 
@@ -664,7 +664,7 @@ listextend(PyListObject *self, PyObject *args)
 	if (!PyArg_ParseTuple(args, "O:extend", &b))
 		return NULL;
 
-	b = PySequence_Fast(b, "list.extend() argument must be a sequence");
+	b = PySequence_Fast(b, "list.extend() argument must be iterable");
 	if (!b)
 		return NULL;