Related to #172, where we discussed that scc_info bindings were
missing.

* spot/twaalgos/sccinfo.hh (spot::scc_info::scc_node): Move...
(spot::scc_info_node): ... here to help Swig.
* python/spot/impl.i: Add bindings for scc_info.
* tests/python/sccinfo.py: New file.
* tests/Makefile.am: Add it.