[Release] Sync master with v0.15 changes
## 0.15.0 (16 Feb 2021)
* Added support for python version 3.9 (#744)
* Added support for 'Get View by ID' (#750)
* Added docs and test data to MANIFEST.in file (#780)
* Added owner_id property to ProjectItem (#784)
* Added support for skipping connection check while publishing workbook (#791)
* Added support for 'Update Subscription' (#794)
* Added support for 'Get Groups for a User' (#799)
* Improved debug logging by including put/post request contents (#743)
* Improved local and active-directory group creation (#770)
* Improved 'Update Group' to match server requests/responses (#772)
* Improved SiteItem with new properties and functions (#777)
* Improved SubscriptionItem with new properties (#794)
* Improved the 'type' property of TaskItem to convert server response to enum (#796)
* Improved repository to use Github Actions for running tests/linter (#798)
* Fixed data_acceleration field causing error in workbook update payload (#741)