Tested on Linux-x64.

2017-03-29  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Adjust optional's pretty printer for LWG 2900.
    * python/libstdcxx/v6/printers.py (StdExpOptionalPrinter::__init__):
    Look at the nested payload in case of non-experimental optional.