providers/*: remove path-based import from all providers

passbook/providers/saml/models.py

@@ -1,7 +1,8 @@
 """passbook saml_idp Models"""
-from typing import Optional
+from typing import Optional, Type
 
 from django.db import models
+from django.forms import ModelForm
 from django.http import HttpRequest
 from django.shortcuts import reverse
 from django.utils.translation import ugettext_lazy as _
@@ -101,7 +102,10 @@ class SAMLProvider(Provider):
         ),
     )
 
-    form = "passbook.providers.saml.forms.SAMLProviderForm"
+    def form(self) -> Type[ModelForm]:
+        from passbook.providers.saml.forms import SAMLProviderForm
+
+        return SAMLProviderForm
 
     def __str__(self):
         return self.name