fix future deprecate warning of django.utils.timezone.utc

[jairhenrique]

Describe the change Make code compatible with future Django release.

model_bakery/timezone.py:6: RemovedInDjango50Warning: The django.utils.timezone.utc alias is deprecated. Please update your code to use datetime.timezone.utc instead.
    from django.utils.timezone import utc

PR Checklist

• [x] Change is covered with tests
• [x] CHANGELOG.md is updated