Add requries-python to pyproject.toml

[gopackgo90]

Adding the Python version requirements of the project helps tools like pip decide if a given version of the package is appropriate to install with a given Python version.

Proposed Changes

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• Bugfix (non-breaking change which fixes issue #NNNN)
• [ x] New feature (non-breaking change which adds functionality)
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• [ x] I have read the CONTRIBUTING.md document
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• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further Comments

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[lukebakken]

Thank you!