Update models.py

[codehero7386]

Brief Summary of Changes

What does this PR address?

• [ ] GitHub issue (Add reference - #XX)
• [ ] Refactoring
• [ ] New feature
• [ ] Bug fix
• [ ] Adds more tests

Are tests included?

• [ ] Yes
• [ ] No

Reviewer, please note:

Checklist:

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I ran the full test suite before pushing the changes and all the tests pass.

[const-cloudinary]

@codehero7386 thank you for contribution! Could you explain a bit what are changes about? Also it looks like all test failed, see for example: https://app.travis-ci.com/github/cloudinary/pycloudinary/jobs/611413196 django.db.utils.OperationalError: no such column: django_tests_poll.created_at

Adding/updating some of those tests would be great!