Improve type hints for UserSerializer

[browniebroke]

Description

I recently came across the fact that djangorestframework-stubs supports using generics in model serializers, which has the effect of improving type hints: self.instance is known to be an instance of the model instead of Any.

It would be nice to show that feature in the only serializer that we include, the UserSerializer.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

Improve discoverability of a nice feature, improve type checking, improve auto-completion.

[foarsitter]

It is good to know about this feature. Do you have some more background on this?

[browniebroke]

I've stumbled upon that in a closed issue on the DRF-stubs and I'm intending to send a pull request to document these better in the upstream project. For now, I'm learning more about them in one of my projects.

[browniebroke]

Unless there is any objections, I'm going to merge this in the next couple of days, it seems pretty safe AFAIK