convert: convert os.devnull to bytes before trying to join it with other bytes
Together with the previous commit relating to emailparser this gets
test-convert-tla.t passing under py3.
Differential Revision: https://phab.mercurial-scm.org/D7062
#include <Python.h>
namespace contrib
{
void initpy(const char *cselfpath);
PyObject *pyglobals();
} /* namespace contrib */