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.
7ac570fa3f