chore: Add tox<4.0.0 to constraints.txt

[ZiqiXiao]

Description: Add tox<4.0.0 to constraints.txt

Reviewers:

• @feanil

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

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[ZiqiXiao]

Add tox<4.0.0 to constraints.txt (public-engineering#217)

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