Edx-api-client

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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.

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- loosen requirements (48)

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- Update grades API to v1 (45)
- Update six version to latest (46)

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- Added timeout in edx requests (40)

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- Specify pylint version
- Lint
- Fix test
- Added user_info client
- Fix lint
- Lint
- Fix code coverage
- Updated client with edX newly added attributes in certificates api (31)