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/Modules/sha256module.c b/Modules/sha256module.c
index da31d18..a5dc9ad 100644
--- a/Modules/sha256module.c
+++ b/Modules/sha256module.c
@@ -432,7 +432,7 @@
 
     SHAcopy(self, &temp);
     sha_final(digest, &temp);
-    return PyBytes_FromStringAndSize((const char *)digest, self->digestsize);
+    return PyString_FromStringAndSize((const char *)digest, self->digestsize);
 }
 
 PyDoc_STRVAR(SHA256_hexdigest__doc__,