Fixes #118.

* spot/twa/twagraph.hh: Avoid using graph_t::state to help Swig.
* wrap/python/spot_impl.i: Add a __str__ function for acc_cond::mark_t.
* doc/org/tut21.org: Add the Python version.
* doc/org/tut.org: Move tut21.org to the Python/C++ section.
* NEWS: Update.