chore: Add tox<4.0.0 to constraints.txt

[ZiqiXiao]

Add tox<4.0.0 to containts.txt for public-engineering#217

Reviewers:

• @feanil

[openedx-webhooks]

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[feanil]

@ZiqiXiao the change looks pretty reasonable, but we need you to sign the CLA. Please let me know once you've completed that work. Usually it takes about 24h on our side to process a CLA.

[ZiqiXiao]

@ZiqiXiao the change looks pretty reasonable, but we need you to sign the CLA. Please let me know once you've completed that work. Usually it takes about 24h on our side to process a CLA.

@feanil I have signed the CLA and it's been processed. I think I am ready.

[openedx-webhooks]

@ZiqiXiao 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.