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_extcall.py

@@ -58,20 +58,20 @@ g(1, 2, 3, *(4, 5))
 class Nothing: pass
 try:
     g(*Nothing())
-except AttributeError, attr:
+except TypeError, attr:
     pass
 else:
-    print "should raise AttributeError: __len__"
+    print "should raise TypeError"
 
 class Nothing:
     def __len__(self):
         return 5
 try:
     g(*Nothing())
-except AttributeError, attr:
+except TypeError, attr:
     pass
 else:
-    print "should raise AttributeError: __getitem__"
+    print "should raise TypeError"
 
 class Nothing:
     def __len__(self):