Since we now require Python 3.6, we can use f-strings instead of
format() to make the code more readable.
* doc/org/tut01.org, doc/org/tut02.org, doc/org/tut03.org,
doc/org/tut21.org, doc/org/tut24.org, doc/org/tut90.org,
python/spot/__init__.py, python/spot/jupyter.py, tests/python/acc.py,
tests/python/acc_cond.ipynb, tests/python/complement_semidet.py,
tests/python/decompose.ipynb, tests/python/formulas.ipynb,
tests/python/highlighting.ipynb, tests/python/ipnbdoctest.py,
tests/python/ltlf.py, tests/python/parity.ipynb,
tests/python/product.ipynb, tests/python/relabel.py,
tests/python/satmin.ipynb, tests/python/stutter-inv.ipynb,
tests/python/twagraph-internals.ipynb, tests/python/zlktree.ipynb: Use
f-strings.
Most of those errors were pointed out by the language-check tool.
However while fixing those I found a few other issues that I fixed.
In particular I updated the bibliographic reference for ltlsynt,
added some DOI links for some cited papers that had no link, and
fixed the broken introduction of ltlgrind.
* doc/org/autcross.org, doc/org/autfilt.org, doc/org/citing.org,
doc/org/compile.org, doc/org/concepts.org, doc/org/csv.org,
doc/org/dstar2tgba.org, doc/org/genaut.org, doc/org/hierarchy.org,
doc/org/install.org, doc/org/ioltl.org, doc/org/ltl2tgba.org,
doc/org/ltl2tgta.org, doc/org/ltlcross.org, doc/org/ltldo.org,
doc/org/ltlfilt.org, doc/org/ltlgrind.org, doc/org/ltlsynt.org,
doc/org/oaut.org, doc/org/randaut.org, doc/org/randltl.org,
doc/org/satmin.org, doc/org/tut01.org, doc/org/tut02.org,
doc/org/tut03.org, doc/org/tut10.org, doc/org/tut11.org,
doc/org/tut12.org, doc/org/tut20.org, doc/org/tut22.org,
doc/org/tut24.org, doc/org/tut30.org, doc/org/tut40.org,
doc/org/tut50.org, doc/org/tut51.org, doc/org/tut52.org,
doc/org/tut90.org, doc/org/upgrade2.org: Fix errors.
* bin/autfilt.cc, bin/common_aoutput.cc, bin/genaut.cc: Fix some
typos in --help text that appeared in the above org files.
