Update factory-boy to 3.2.0

[pyup-bot]

This PR updates factory-boy from 3.1.0 to 3.2.0.

Changelog

3.2.0

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

*New:*

 - Add support for Django 3.1
 - Add support for Python 3.9

*Removed:*

 - Drop support for Django 1.11. This version `is not maintained anymore <https://www.djangoproject.com/download/supported-versions>`__.
 - Drop support for Python 3.5. This version `is not maintained anymore <https://devguide.python.org/devcycle/end-of-life-branches>`__.

*Deprecated:*

 - :func:`factory.use_strategy`. Use :attr:`factory.FactoryOptions.strategy` instead.
   The purpose of :func:`~factory.use_strategy` duplicates the factory option. Follow :pep:`20`: *There should be
   one-- and preferably only one --obvious way to do it.*

   :func:`~factory.use_strategy()` will be removed in the next major version.

*Bug fix:*

 - :issue:`785` :issue:`786` :issue:`787` :issue:`788` :issue:`790` :issue:`796`: Calls to :class:`factory.Faker`
   and :class:`factory.django.FileField` within a :class:`~factory.Trait` or :class:`~factory.Maybe` no longer lead to
   a ``KeyError`` crash.

Links

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

[gafderks]

Cannot be merged as the Faker object does not offer the generate method anymore. Therefore it cannot be used to generate unique values. Follow FactoryBoy/factory_boy#820