Take namedtuple item names only from ascii_letters (this blew up on OSX),

and make sure there are no duplicate names.

Lib/test/test_collections.py

@@ -110,7 +110,9 @@ class TestNamedTuple(unittest.TestCase):
 
         n = 10000
         import string, random
-        names = [''.join([random.choice(string.letters) for j in range(10)]) for i in range(n)]
+        names = list(set(''.join([random.choice(string.ascii_letters)
+                                  for j in range(10)]) for i in range(n)))
+        n = len(names)
         Big = namedtuple('Big', names)
         b = Big(*range(n))
         self.assertEqual(b, tuple(range(n)))