Add type annotations

Problem

When type-checking a model that uses InheritanceManager using mypy, the following error is displayed:

app/models.py:39: error: Could not resolve manager type for
"app.models.MyModel.objects"  [django-manager-missing]
        objects = InheritanceManager()
        ^

According to this Stack Overflow post, the origin of this problem is the django-stubs mypy plugin being unable to find InheritanceManager because django-model-utils does not provide type annotations.

According to the linked post, no actual annotations are required, just a marker that the package is annotated. But I think real annotations would be good to have anyway, both for users and developers of django-model-utils.

I am working on adding annotations to django-model-utils. I don't know if I'll have enough time to completely annotate it, but I'll share what I have in any case, probably tomorrow.

Environment

• Django Model Utils version: 4.3.1
• Django version: 3.2
• Python version: 3.9
• Other libraries used, if any: django-stubs 1.16.0

The issue names no repository files or tests beyond the example app/models.py and InheritanceManager entry point. Start by reproducing the django-stubs mypy error with that model, then inspect django-model-utils' public utilities and existing test coverage. Done means the package is recognized as typed and the agreed annotation scope is covered without breaking supported Django and Python versions.