tests: Update tox.ini versions list to match CI

[EliahKagan]

Currently, CI runs tests on Python 3.7-3.11, but tox.ini still lists the older range 3.6-3.8.

That affects local development, because running tox does not test with 3.9, 3.10, or 3.11, even when those Python versions are installed on the local development system. (I noticed this when locally testing #238.) Less importantly, it is probably not necessary to test on Python 3.6 anymore, because it has been unsupported for a long time.

This PR changes the Python version range in tox.ini to match the range tested on CI (in the build step in pythonpackage.yml).

Although Python 3.7 is recently unsupported as well, I have not removed it in this PR. If 3.7 is to be removed from tox.ini, then that should also be done in pythonpackage.yml, and it may make sense to update some other things there at the same time. I'd be pleased to make any requested changes to this PR, but my current thinking is that I should keep it narrowly scoped to tox, and open a separate PR later for changes that include modifying pythonpackage.yml.

I have tested these changes locally by running tox on an Ubuntu 22.04.3 LTS system with Python 3.7.17, 3.8.17, 3.9.17, 3.10.12 and 3.11.4 installed.

[cwacek]

Good call all around. Thanks for the update. All other things being equal, I'd like to keep 3.7 since I suspect there are a lot of systems still using it, unlike 3.6