Migrate from tox to nox

[kiudee]

Description

This pull request removes tox and implements the same functionality in nox.

Motivation and Context

Tox was a versatile framework, but the configuration format was hard to work with. In nox we can specify the configuration as python code, which makes it much more natural to work with and maintain.

How Has This Been Tested?

Running nox locally and on GitHub Actions.

Does this close/impact existing issues?

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (gh-actions@2535707). Click here to learn what that means. The diff coverage is n/a.

@@              Coverage Diff              @@
##             gh-actions     #190   +/-   ##
=============================================
  Coverage              ?   52.14%           
=============================================
  Files                 ?      102           
  Lines                 ?     5090           
  Branches              ?        0           
=============================================
  Hits                  ?     2654           
  Misses                ?     2436           
  Partials              ?        0