Add type hint for UserFactory

[browniebroke]

Description

Since factory-boy 3.3.1, we can type annotate factories:

FactoryBoy/factory_boy#1059

Setting that up for the only factory of the project

Checklist:

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

Rationale

Demonstrate best practice as part of the project

[foarsitter]

This is awesome!