Get Python unit tests working with Python 3.

There were a couple of issues related to string handling that
needed to be fixed.  In particular, we cannot get away with
converting `PyUnicode` objects to `PyBytes` objects and storing
the `PyBytes` regardless of Python version.  Instead we have to
store a `PyUnicode` on Python 3 and a `PyString` on Python 2.

The reason for this is that if you call `PyObject_Str` on a
`PyBytes` in Python 3, it will return you a string that actually
contains the string value wrappedin the characters b''.  So if we
create a `PythonString` with the value "test", and we call Str()
on it, we will get back the string "b'test'", which breaks string
equality.  The only way to fix this is to store a native
`PyUnicode` object under Python 3.

With this CL, ScriptInterpreterPythonTests unit tests pass 100%
under Python 2 and Python 3.

llvm-svn: 250327