Generalize tuple() to work nicely with iterators.

NEEDS DOC CHANGES.
This one surprised me!  While I expected tuple() to be a no-brainer, turns
out it's actually dripping with consequences:
1. It will *allow* the popular PySequence_Fast() to work with any iterable
   object (code for that not yet checked in, but should be trivial).
2. It caused two std tests to fail.  This because some places used
   PyTuple_Sequence() (the C spelling of tuple()) as an indirect way to test
   whether something *is* a sequence.  But tuple() code only looked for the
   existence of sq->item to determine that, and e.g. an instance passed
   that test whether or not it supported the other operations tuple()
   needed (e.g., __len__).  So some things the tests *expected* to fail
   with an AttributeError now fail with a TypeError instead.  This looks
   like an improvement to me; e.g., test_coercion used to produce 559
   TypeErrors and 2 AttributeErrors, and now they're all TypeErrors.  The
   error details are more informative too, because the places calling this
   were *looking* for TypeErrors in order to replace the generic tuple()
   "not a sequence" msg with their own more specific text, and
   AttributeErrors snuck by that.

Lib/test/test_iter.py

@@ -275,6 +275,39 @@ class TestCase(unittest.TestCase):
             except OSError:
                 pass
 
+    # Test tuples()'s use of iterators.
+    def test_builtin_tuple(self):
+        self.assertEqual(tuple(SequenceClass(5)), (0, 1, 2, 3, 4))
+        self.assertEqual(tuple(SequenceClass(0)), ())
+        self.assertEqual(tuple([]), ())
+        self.assertEqual(tuple(()), ())
+        self.assertEqual(tuple("abc"), ("a", "b", "c"))
+
+        d = {"one": 1, "two": 2, "three": 3}
+        self.assertEqual(tuple(d), tuple(d.keys()))
+
+        self.assertRaises(TypeError, tuple, list)
+        self.assertRaises(TypeError, tuple, 42)
+
+        f = open(TESTFN, "w")
+        try:
+            for i in range(5):
+                f.write("%d\n" % i)
+        finally:
+            f.close()
+        f = open(TESTFN, "r")
+        try:
+            self.assertEqual(tuple(f), ("0\n", "1\n", "2\n", "3\n", "4\n"))
+            f.seek(0, 0)
+            self.assertEqual(tuple(f.xreadlines()),
+                             ("0\n", "1\n", "2\n", "3\n", "4\n"))
+        finally:
+            f.close()
+            try:
+                unlink(TESTFN)
+            except OSError:
+                pass
+
     # Test filter()'s use of iterators.
     def test_builtin_filter(self):
         self.assertEqual(filter(None, SequenceClass(5)), range(1, 5))