Update factory_boy to 2.12.0

[pyup-bot]

This PR updates factory_boy from 2.11.1 to 2.12.0.

Changelog

2.12.0

-------------------

*New:*

 - Add support for Python 3.7
 - Add support for Django 2.1
 - Add :attr:`~factory.fuzzy.FuzzyChoice.getter` to :class:`~factory.fuzzy.FuzzyChoice` that mimics
   the behavior of ``getter`` in :class:`~factory.Iterator`
 - Make the ``extra_kwargs`` parameter of :meth:`~factory.faker.Faker.generate` optional
 - Add :class:`~factory.RelatedFactoryList` class for one-to-many support, thanks `Sean Harrington <https://github.com/seanharr11>`_.
 - Make the `locale` argument for :class:`~factory.faker.Faker` keyword-only

*Bugfix:*

 - Allow renamed arguments to be optional, thanks to `Justin Crown <https://github.com/mrname>`_.
 - Fix `django_get_or_create` behavior when using multiple fields with `unique=True`, thanks to `YPCrumble <https://github.com/YPCrumble>`

Links

• PyPI: https://pypi.org/project/factory-boy
• Changelog: https://pyup.io/changelogs/factory-boy/
• Repo: https://github.com/FactoryBoy/factory_boy