cxxtest: use venv

[branchvincent]

• Have you followed the guidelines for contributing?
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• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
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We don't want to expose python packages globally for CLI-only formulae

[github-actions]

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