chore: Replicate Django's signature at SoftDelete queryset

[gmcrocetti]

Problem

Mimics Django's API for SoftDeletableQuerySetMixin.

Solution

Django's API behaves as the following:

Returning a tuple of type (int, dict[str, int]) where the first element is the number of objects deleted and the second the same count but for the model itself and its relations.

The proposed solution returns the number of objects affected by the update and a made up dict where the key is model's label (no relations as we don't touch it) with aforementioned count.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-541.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).

[foarsitter]

Thanks @gmcrocetti !