Why does this lock my table for reading? How do I make it NOT lock the table?

evil_migration.py

# -*- coding: utf-8 -*-
# Generated by Django 1.10.5 on 2017-03-29 16:51
from __future__ import unicode_literals

from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
    ]

    operations = [
        migrations.AddField(
            model_name='mymodel',
            name='newfield',
            field=models.TextField(blank=True, null=True),
        ),
    ]
# -- Translates into this SQL --
# BEGIN;
#  ALTER TABLE "app.mymodel" ADD COLUMN "newfield" text NULL;
#  ALTER TABLE "app.mymodel" ALTER COLUMN "newfield" DROP DEFAULT;
# COMMIT;