Get a tag-length-value encoded attribute.

def TLV_GET(attrs, attrNum, format):
    """ Get a tag-length-value encoded attribute. """
    attrView = attrs[attrNum]
    if format == 's':
        format = str(attrView.len) + format
    try:
        (result,) = struct.unpack_from(format, attrView.buf, attrView.offset)
    except TypeError:
        # Working around struct.unpack_from issue #10212
        (result,) = struct.unpack_from(format, str(bytearray(attrView.buf)), attrView.offset)
    return result