chore(pyproject): quote versions

[mr-cal]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

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X.Y versions in a pyproject.toml need to be quoted to avoid being treated as an integer to avoid warnings like this:

 pyproject.toml: [mypy]: python_version: Python 3.1 is not supported (must be 3.4 or higher). You may need to put quotes around your Python version