dependencies: Upgrade to Django 3.1.

https://docs.djangoproject.com/en/3.1/releases/3.1/

- django.contrib.postgres.fields.JSONField is deprecated and should be
  replaced with models.JSONField
-  The internals of the implementation in the postgresql backend have
   changed a bit in
   f48f671223
   and thus we need to make an ugly tweak in test_runner.
- app_directories.Loader.get_dirs() now returns a list of PosixPath so
  we need to make a small tweak in TwoFactorLoader for that (PosixPath
  is not iterable)

Fixes #16010.

zerver/migrations/0310_jsonfield.py

@@ -0,0 +1,24 @@
+# Generated by Django 3.1.5 on 2021-01-10 11:30
+
+from django.db import migrations, models
+
+
+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.
+    """
+
+    dependencies = [
+        ('zerver', '0309_userprofile_can_create_users'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='userprofile',
+            name='zoom_token',
+            field=models.JSONField(default=None, null=True),
+        ),
+    ]