Django-sendgrid-v5

Improves flexibility of personalizations by allowing the user to set the `EmailMessage.personalizations` attribute to a list of sendgrid `Personalization` objects, or a list of Dicts containing personalization information in the schema expected by the API. Personalization options passed this way take precedence over other EmailMessage attributes.

Breaking Changes
- Drops python 2 support

Improvements
- Adds type checking, linting, and code formatting
- Simplifies code where possible (no functionality changes)
- Adds github actions support
- Adds newer django versions to the testing matrix
- Improves documentation

Deprecations
- Removes TravisCI support
- Removes codecov uploads (coverage is still run in the tests)

Breaking Changes
- **Drops python 2 support**

Improvements
- Adds type checking, linting, and code formatting
- Simplifies code where possible (no functionality changes)
- Adds github actions support
- Adds newer django versions to the testing matrix
- Improves documentation

Deprecations
- Removes TravisCI support
- Removes codecov uploads (coverage is still run in the tests)

Thanks to MattFanto for the PR and bug fix!

Now supports the newest version of sendgrid's api (https://github.com/sendgrid/sendgrid-python/releases/tag/v6.0.0). Maintains backwards compatibility.