Django-sendgrid-v5

Fix 114

Adds attachment content-type method handling (103)
Adds configurable tracking per-email (101)

Adds Django 3.2+ & 4.0 compatibility

Closes https://github.com/sklarsa/django-sendgrid-v5/issues/99

Personalization Fixes:
- _build_sg_personalization "to" arg now changed to optional; either it or existing_personalizations must be set.
- Prevent using .personalizations without tos set.
- Prevent unnecessary resetting of tos when personalization is already constructed.
- No longer provide tos if not set for personalizations. This is to prevent every personalization from being sent to every recipient in msg.to
- Added/Changed some comments/docstrings to reduce ambiguity

Thanks alichass for the contribution!

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)