It seems safe to bump our requirements and require Python 3.8.  This
is what Ubuntu 20.04 uses, which is the oldest distribution we compile
for at the moment.  Both Python 3.8 and Ubuntu 20.04 have reached
end-of-life.

* NEWS: Mention the bumped requirements.
* m4/pypath.m4, HACKING, README: Bump the requirement.
* doc/org/tut25.org, doc/org/tut90.org, python/spot/__init__.py,
python/spot/aux_.py, python/spot/ltsmin.i: Modernize.
* tests/python/_aux.ipynb: Adjust.