chore: Add tox<4.0.0 to constraints.txt

[ZiqiXiao]

Run make upgrade and files are updated.

Reviewers:

• @feanil

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[feanil]

You need to run make upgrade with Python 3.8 other wise this could pull in package versions that are not compatible with 3.8 and break current deployments.

[ZiqiXiao]

I have rerun make upgrade with Python 3.8, please review the commit.

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