migrations: Swap out deprecated JSONField import in-place.

A deprecated import shouldn’t be used even in a migration, since the
migration will need to remain runnable in the future.  We never needed
a migration for this switch anyway; we just needed to edit the old
migration, since no actual state changes are involved.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/migrations/0281_zoom_oauth.py

@@ -1,7 +1,6 @@
 # Generated by Django 1.11.25 on 2019-11-06 22:40
 
-import django.contrib.postgres.fields.jsonb
-from django.db import migrations
+from django.db import migrations, models
 
 
 class Migration(migrations.Migration):
@@ -14,6 +13,6 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name="userprofile",
             name="zoom_token",
-            field=django.contrib.postgres.fields.jsonb.JSONField(default=None, null=True),
+            field=models.JSONField(default=None, null=True),
         ),
     ]

zerver/migrations/0310_jsonfield.py

@@ -1,24 +1,17 @@
 # Generated by Django 3.1.5 on 2021-01-10 11:30
 
-from django.db import migrations, models
+from typing import List
+
+from django.db import migrations
 
 
 class Migration(migrations.Migration):
     """
-    This doesn't actually run any SQL, it's for Django's internal
-    tracking of changes to models only.
-    django.contrib.postgres.fields.JSONField is deprecated as of Django 3.1
-    and should be replaced by models.JSONField which offers the same functionality.
+    This empty migration is only retained for Django's tracking.
     """
 
     dependencies = [
         ("zerver", "0309_userprofile_can_create_users"),
     ]
 
-    operations = [
-        migrations.AlterField(
-            model_name="userprofile",
-            name="zoom_token",
-            field=models.JSONField(default=None, null=True),
-        ),
-    ]
+    operations: List[migrations.operations.base.Operation] = []