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

src/drf_yasg/generators.py

@@ -409,9 +409,9 @@ class OpenAPISchemaGenerator(object):
             if getattr(view_cls, 'lookup_field', None) == variable and attrs['type'] == openapi.TYPE_STRING:
                 attrs['pattern'] = getattr(view_cls, 'lookup_value_regex', attrs.get('pattern', None))
 
-            if model_field and model_field.help_text:
+            if model_field and getattr(model_field, 'help_text', False):
                 description = force_text(model_field.help_text)
-            elif model_field and model_field.primary_key:
+            elif model_field and getattr(model_field, 'primary_key', False):
                 description = get_pk_description(model, model_field)
             else:
                 description = None