Redundant index on AccessAttempt.username

[GougeGouge]

I was doing some Index analysis on my Platform and noticed AccessAttempt has a redundant index on username as already covered by unique_together = ("username", "ip_address", "user_agent").

I know it's pretty much nothing and probably micro-optimization but if you want i can create a PR fix it.

Mainly consist on adding:

username = models.CharField(_("Username"), max_length=255, null=True)`

to the model AccessAttempt to overide username without db_index=True.

Migration generated by this fix

from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ("axes", "0010_accessattemptexpiration"),
    ]

    operations = [
        migrations.AlterField(
            model_name="accessattempt",
            name="username",
            field=models.CharField(max_length=255, null=True, verbose_name="Username"),
        ),
    ]

NB: I use PostgreSQL and don't have the knowledge on other Database behavior