Until Tutor 17, just enabling a plugin with "tutor plugin enable"
would not recreate the configuration, meaning the
tests/tutor-sandbox/env directory would remain empty.

As of overhangio/tutor#957, however,
"tutor plugin enable" does automatically also run "tutor config save",
generating a full Tutor configuration.

This means that unless configured otherwise, with Tutor 17 our flake8
checks would test code generated by Tutor, rather than just that
generated by our plugin.

Thus, to make our tests not break once we bump our dependencies to
support Tutor 17, exclude the tests/tutor-sandbox/env checks from
flake8.

* Update the install_requires list to support Tutor 17.
* Update the compatibility matrix in the README accordingly.