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.

Lib/test/testcodec.py

@@ -36,7 +36,7 @@ def getregentry():
 decoding_map = codecs.make_identity_dict(range(256))
 decoding_map.update({
         0x78: "abc", # 1-n decoding mapping
-        str8(b"abc"): 0x0078,# 1-n encoding mapping
+        b"abc": 0x0078,# 1-n encoding mapping
         0x01: None,   # decoding mapping to <undefined>
         0x79: "",    # decoding mapping to <remove character>
 })