Add Tox plugin to work around cyclic dependency on verifai

[dfremont]

Description

The tox environments with the test-full extra (py38-extras etc.) recently broke, since they install verifai, whose cyclic dependency on scenic causes issues with the way that tox installs dependencies (see discussion here). Until we get a better solution, this PR adds a Tox plugin which works around the problem.

Issue Link

n/a

Checklist

• [x] I have tested the changes locally via pytest and/or other means
• [ ] I have added or updated relevant documentation
• [x] I have autoformatted the code with black and isort
• [ ] I have added test cases (if applicable)

Additional Notes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.12%. Comparing base (d846a15) to head (6643011).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #251 +/- ## ========================================== + Coverage 86.10% 86.12% +0.01% ========================================== Files 144 144 Lines 24984 24986 +2 ========================================== + Hits 21512 21518 +6 + Misses 3472 3468 -4 ```

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[Eric-Vin]

Looks good to me, though I hope we can get a permanent solution soon.