I've converted _bsddb.c to use the py3k buffer API for all data and keys
it takes as input.  All tests now fail with this error:

BufferError: Cannot make this object read-only.

This presumably results from this call:

  PyObject_GetBuffer(obj, view, PyBUF_LOCKDATA)

I need to lock the data so that the GIL can be released during database
operations (I/O).

Allowing bytes objects to have an immutability or readonly bit (internal
or otherwise) has been a recent topic on the python-3000 mailing list;
that would allow bytes' buffer API to satisfy this GetBuffer LOCKDATA
request...