Remove mentions of Tox

[akx]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
  • Unsurprisingly, no, since there is no Tox configuration.
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?
  • No, since the CHANGES file is currently very out of date.

Description of change

The PR checklist and other documentation refers to tox, but it was removed in #2849.