Fix schema generation with OneToOneFields (#81)

* Fix: OneToOneRel, used by OneToOneField doesn't have help_text nor primary_key attributes, thus breaking OpenAPISchemaGenerator; use hasattr() as safe-guard.
* Fix: use getattr() with a default value instead of hasattr() + acessing the value
* Add: 'people' app that breaks drf_yasg without previous commits
* Update tests/references.yaml + run isort and flake8
* Fix: set on_delete for Person.identity as Django-2+ requires it

testproj/testproj/settings/base.py

@@ -27,6 +27,7 @@ INSTALLED_APPS = [
     'users',
     'articles',
     'todo',
+    'people'
 ]
 
 MIDDLEWARE = [

testproj/testproj/urls.py

@@ -63,5 +63,6 @@ urlpatterns = [
     url(r'^articles/', include('articles.urls')),
     url(r'^users/', include('users.urls')),
     url(r'^todo/', include('todo.urls')),
+    url(r'^people/', include('people.urls')),
     url(r'^plain/', plain_view),
 ]