Flow exporting/importing (#187)

* stages/*: Add SerializerModel as base model, implement serializer property

* flows: add initial flow exporter and importer

* policies/*: implement .serializer for all policies

* root: fix missing dacite requirement

passbook/policies/expiry/models.py

@@ -6,6 +6,7 @@ from django.db import models
 from django.forms import ModelForm
 from django.utils.timezone import now
 from django.utils.translation import gettext as _
+from rest_framework.serializers import BaseSerializer
 from structlog import get_logger
 
 from passbook.policies.models import Policy
@@ -21,6 +22,12 @@ class PasswordExpiryPolicy(Policy):
     deny_only = models.BooleanField(default=False)
     days = models.IntegerField()
 
+    @property
+    def serializer(self) -> BaseSerializer:
+        from passbook.policies.expiry.api import PasswordExpiryPolicySerializer
+
+        return PasswordExpiryPolicySerializer
+
     def form(self) -> Type[ModelForm]:
         from passbook.policies.expiry.forms import PasswordExpiryPolicyForm