In pure Python, _dict_to_bson simply returns the RawBSONDocument.raw attribute:

def _dict_to_bson(doc, check_keys, opts, top_level=True):
    """Encode a document to BSON."""
    if _raw_document_class(doc):
        return doc.raw
    ...

In the C version _cbson_dict_to_bson, we copy the raw attribute to an intermediate buffer and then encode that buffer to a bytes instance. Let's just return the raw attribute the same way the python code does instead.