fix: guard constraints setdefault behind Django >= 2.2 check

Django < 2.2 does not support the constraints Meta option, so setting
it as a default causes "'class Meta' got invalid attribute(s): constraints".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: armonge

psqlextra/backend/migrations/operations/create_materialized_view_model.py

@@ -1,3 +1,4 @@
+import django
 from django.db.migrations.operations.models import CreateModel
 
 from psqlextra.backend.migrations.state import (
@@ -34,7 +35,8 @@ def __init__(
     def state_forwards(self, app_label, state):
         options = dict(self.options)
         options.setdefault("indexes", [])
-        options.setdefault("constraints", [])
+        if django.VERSION >= (2, 2):
+            options.setdefault("constraints", [])
         state.add_model(
             PostgresMaterializedViewModelState(
                 app_label=app_label,

psqlextra/backend/migrations/operations/create_partitioned_model.py

@@ -1,3 +1,4 @@
+import django
 from django.db.migrations.operations.models import CreateModel
 
 from psqlextra.backend.migrations.state import PostgresPartitionedModelState
@@ -29,7 +30,8 @@ def __init__(
     def state_forwards(self, app_label, state):
         options = dict(self.options)
         options.setdefault("indexes", [])
-        options.setdefault("constraints", [])
+        if django.VERSION >= (2, 2):
+            options.setdefault("constraints", [])
         state.add_model(
             PostgresPartitionedModelState(
                 app_label=app_label,

psqlextra/backend/migrations/operations/create_view_model.py

@@ -1,3 +1,4 @@
+import django
 from django.db.migrations.operations.models import CreateModel
 
 from psqlextra.backend.migrations.state import PostgresViewModelState
@@ -29,7 +30,8 @@ def __init__(
     def state_forwards(self, app_label, state):
         options = dict(self.options)
         options.setdefault("indexes", [])
-        options.setdefault("constraints", [])
+        if django.VERSION >= (2, 2):
+            options.setdefault("constraints", [])
         state.add_model(
             PostgresViewModelState(
                 app_label=app_label,