Add isort as an dev requirement.

requirements-dev.txt

@@ -4,6 +4,7 @@ django-debug-toolbar==2.2
 factory_boy==2.12
 Faker==1.0.8  # Pinning to avoid API changes to pydecimal in 0.x - could go higher in future
 flake8==3.7.9
+isort==4.3.21
 responses==0.10.9
 stellar==0.4.5
 wagtail-factories==2.0.0