Add set_milestone.py

[jennyhickson]

PR Summary

Sci/Tech Reviewer:
Code Reviewer: @james-bruten-mo

Extract the function to set all closed pull requests to a milestone into its own script as this is a useful thing to run periodically standalone. finish_milestone.py then imports this routine to make use of it.

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Sci/Tech Review

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Code Review

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[james-bruten-mo]

Sensible to be able to run this on its own