Edx-api-client

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- Replace Travis with Github actions (73)

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- Added API to update user name and removed pep8

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- Update CourseDetails client to pass username

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- Added unenrollment functionality

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- Allow for setting api_key to the edX API key

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- Allow to specify enrollment mode when creating enrollment for student
- Add functionality to query grades by course_id.
- Implement interface to call enrollments list from edx-platform. The new `edx_api.enrollments.CourseEnrollments.get_enrollments` method will return all the enrollments for one or multiple course_ids.