add Cython build requirement to pyproject.toml

[svandiekendialpad]

What does this PR do ?

Add Cython as a build requirement to pyproject.toml.

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Additional Information

This is related to #6854 and solves the issue for me in Python 3.9 and Python 3.10. Instead of requiring the user to manually install cython beforehand, why not specify it as a build requirement as per PEP-0518.

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