Python - not snake-casing some property names

[Ndiritu]

There's a few edge cases of properties in Graph models that are not snake cased correctly.

e.g.

• mailboxlocations and sitelocations in SubjectRightsRequest
• externalcourse_activity_id in LearningCourseActivity

[baywet]

In the case of externalcourse_activity_id, this is "normal" as the property is named "externalcourseActivityId" (notice the lowercased c) on the service.
For the others, I agree that we're missing some underscores. Especially when we consider that the casing is right in other languages

[sebastienlevert]

Let's be careful as this would be a breaking change for future versions of the Graph SDK. @shemogumbe I feel this should be fixed before GA.