Enhancement: Update mentorship_relation.py to use enum class

[ciheanyi]

Fixes issue #1143. Code within isValidState function now uses an enum class when completing check.

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[ciheanyi]

Hi just dropping this again for a review for the pull request. I also think there may be some dependency updates required that are causing the unit tests to fail. I believe the way markup is imported may have recently deprecated for some jinja2 versions. When we modified dependency locally, unit tests ran successfully. Created bug here