* tests/python/ipnbdoctest.py: Adjust to Python <3.6.
* tests/python/_autparserr.ipynb: Adjust to older IPython version.
* tests/python/stutter-inv.ipynb: Avoid pandas because its output
varies from version to version.
Fixes#311.
* tests/python/ipnbdoctest.py: Adjust to process the new format,
with a lot of inspiration from Vcsn's copy of this file.
* tests/python/_altscc.ipynb, tests/python/_aux.ipynb,
tests/python/acc_cond.ipynb, tests/python/accparse.ipynb,
tests/python/alternation.ipynb, tests/python/atva16-fig2a.ipynb,
tests/python/atva16-fig2b.ipynb, tests/python/automata-io.ipynb,
tests/python/automata.ipynb, tests/python/decompose.ipynb,
tests/python/formulas.ipynb, tests/python/gen.ipynb,
tests/python/highlighting.ipynb, tests/python/ltsmin-dve.ipynb,
tests/python/ltsmin-pml.ipynb, tests/python/parity.ipynb,
tests/python/piperead.ipynb, tests/python/product.ipynb,
tests/python/randaut.ipynb, tests/python/randltl.ipynb,
tests/python/stutter-inv.ipynb, tests/python/testingaut.ipynb,
tests/python/word.ipynb: Upgrade to the new format.
* NEWS: Mention the change.
* spot/twaalgos/stutter.cc,
spot/twaalgos/stutter.hh (stutter_invariant_letters)
(stutter_invariant_states): Get rid of the broken local variant.
* tests/python/stutter-inv.ipynb, NEWS: Document.
* python/spot/impl.i: Bind vector<bdd>.
