Enable postponed evaluation of annotations for all source modules

This allows using the latest annotation syntax supported by the type
checker regardless of the runtime Python version.

model_utils/choices.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import copy
 
 

model_utils/fields.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import secrets
 import uuid
 

model_utils/managers.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import warnings
 
 from django.core.exceptions import ObjectDoesNotExist

model_utils/models.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from django.core.exceptions import ImproperlyConfigured
 from django.db import models
 from django.db.models.functions import Now

model_utils/tracker.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from copy import deepcopy
 from functools import wraps