In setup.py and in README we declare that we support Python >= 3.6, but the nox test sessions only use Python up to 3.8. Python 3.9 is also missing from the classifiers list in setup.py.

We should bump the maximum versions to 3.9 where applicable. If tests fail in 3.9, we should bound the python_requires version in setup.py until the failures are fixed (though - bound it in any case and manually increase only after we confirm that it works with the new minor Python release?).

If we first need to setup Kokoro environment to support Python 3.9 test environment, let's do that and keep this ticket open as a reminder.

cc: @tswast @busunkim96