Retrieve parse definition from the format string `fmt`.

def get_convert_dict(fmt):
    """Retrieve parse definition from the format string `fmt`."""
    convdef = {}
    for literal_text, field_name, format_spec, conversion in formatter.parse(fmt):
        if field_name is None:
            continue
        # XXX: Do I need to include 'conversion'?
        convdef[field_name] = format_spec
    return convdef