Merging the py3k-pep3137 branch back into the py3k branch.
No detailed change log; just check out the change log for the py3k-pep3137
branch.  The most obvious changes:

  - str8 renamed to bytes (PyString at the C level);
  - bytes renamed to buffer (PyBytes at the C level);
  - PyString and PyUnicode are no longer compatible.

I.e. we now have an immutable bytes type and a mutable bytes type.

The behavior of PyString was modified quite a bit, to make it more
bytes-like.  Some changes are still on the to-do list.
diff --git a/Lib/encodings/punycode.py b/Lib/encodings/punycode.py
index f08c807..56e6958 100644
--- a/Lib/encodings/punycode.py
+++ b/Lib/encodings/punycode.py
@@ -10,7 +10,7 @@
 
 def segregate(str):
     """3.1 Basic code point segregation"""
-    base = b""
+    base = buffer()
     extended = set()
     for c in str:
         if ord(c) < 128:
@@ -18,7 +18,7 @@
         else:
             extended.add(c)
     extended = sorted(extended)
-    return (base, extended)
+    return bytes(base), extended
 
 def selective_len(str, max):
     """Return the length of str, considering only characters below max."""
@@ -78,13 +78,13 @@
 digits = b"abcdefghijklmnopqrstuvwxyz0123456789"
 def generate_generalized_integer(N, bias):
     """3.3 Generalized variable-length integers"""
-    result = b""
+    result = buffer()
     j = 0
     while 1:
         t = T(j, bias)
         if N < t:
             result.append(digits[N])
-            return result
+            return bytes(result)
         result.append(digits[t + ((N - t) % (36 - t))])
         N = (N - t) // (36 - t)
         j += 1
@@ -107,13 +107,13 @@
 def generate_integers(baselen, deltas):
     """3.4 Bias adaptation"""
     # Punycode parameters: initial bias = 72, damp = 700, skew = 38
-    result = b""
+    result = buffer()
     bias = 72
     for points, delta in enumerate(deltas):
         s = generate_generalized_integer(delta, bias)
         result.extend(s)
         bias = adapt(delta, points==0, baselen+points+1)
-    return result
+    return bytes(result)
 
 def punycode_encode(text):
     base, extended = segregate(text)