Fix swig typemap for SBEvent.

This needs to be able to handle bytes, strings, and bytearray objects.
In Python 2 this was easy because bytes and strings are the same thing,
but in Python 3 the 2 cases need to be handled separately.  So as not
to mix raw Python C API code with PythonDataObjects code, I've also
introduced a PythonByteArray class to PythonDataObjects to make the
paradigm used here consistent.

llvm-svn: 258741