Add Invoice.get_variant_choices

aleksis/apps/tezor/models/invoice.py

+from django.conf import settings
 from django.contrib.contenttypes.fields import GenericForeignKey
 from django.contrib.contenttypes.models import ContentType
 from django.db import models
@@ -50,6 +51,13 @@ class Invoice(BasePayment, PureDjangoModel):
     for_object_id = models.PositiveIntegerField()
     for_object = GenericForeignKey("for_content_type", "for_object_id")
 
+    @classmethod
+    def get_variant_choices(cls):
+        choices = []
+        for variant in settings.PAYMENT_VARIANTS.keys():
+            choices.append((variant, cls.VARIANT_DISPLAY[self.variant][0]))
+        return choices
+
     def get_variant_name(self):
         return self.__class__.VARIANT_DISPLAY[self.variant][0]