Bump factory-boy from 2.12.0 to 3.1.0 in /src/requirements (#668)

Bumps [factory-boy](https://github.com/FactoryBoy/factory_boy) from 2.12.0 to 3.1.0.
- [Release notes](https://github.com/FactoryBoy/factory_boy/releases)
- [Changelog](https://github.com/FactoryBoy/factory_boy/blob/master/docs/changelog.rst)
- [Commits](https://github.com/FactoryBoy/factory_boy/compare/2.12.0...3.1.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

src/requirements/dev.txt

@@ -1,7 +1,7 @@
 -r production.txt
 
 django-debug-toolbar==2.2
-factory_boy==2.12.0
+factory_boy==3.1.0
 coverage==5.2.1
 codecov==2.1.10
 black==19.10b0