Username: disallow @ in usernames.

myauth/migrations/0002_auto_20200515_0801.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.0.3 on 2020-05-15 08:01
+
+from django.db import migrations, models
+import myauth.models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('myauth', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='user',
+            name='username',
+            field=models.CharField(error_messages={'unique': 'A user with that username already exists.'}, help_text='Required. 150 characters or fewer. Letters, digits and ./+/-/_ only.', max_length=150, unique=True, validators=[myauth.models.UnicodeUsernameValidator()], verbose_name='username'),
+        ),
+    ]