InheritanceManager queries always return an instance of InheritanceQuerySet

## Problem

I am using a custom model manager that extends `InheritanceManager` like so:

```
class Manager(InheritanceManager):
def custom_method(self):
return self.filter(...)
```

because the return type of `get_queryset()` of `InheritanceManagerMixin` is hardcoded to return `self._queryset_class(self.model)` it's impossible for me to chain the methods of my custom `Manager`:

`Model.objects.all().custom_method()` will fail since `.all()` will return an instance of `InheritanceQuerySet` and my `custom_method` isn't defined there, it's rather defined on `Manager`.

Although this slight confusion is mentioned in the documentation [here](https://django-model-utils.readthedocs.io/en/latest/managers.html#mixins) it's still not clear for the user how to properly extend `InheritanceManager` with custom managers

## Environment

- Django Model Utils version: 4.1.1
- Django version: 3.2.11
- Python version: 3.8

## Code examples

`Model.objects.custom_method()` works
`Model.objects.filter().custom_method()` fails with exception `'InheritanceQuerySet' object has no attribute 'custom_method'`

Start with InheritanceManagerMixin.get_queryset() and the custom Manager example in the issue. Reproduce the difference between Model.objects.custom_method() and Model.objects.filter().custom_method() on Django 3.2. Done means chained querysets preserve access to the custom manager method without breaking existing InheritanceManager behavior.

Written by the indexing model from the issue text.