The OP was using the GeoDjango buildpack, but in case anyone gets here using Geo buildpack and dj_database_url like I was, in settings.py don’t forget the last line:

import dj_database_url
DATABASES['default'] = dj_database_url.config()
DATABASES['default']['ENGINE'] = 'django.contrib.gis.db.backends.postgis'

UPDATE

dj_database_url directly supports PostGIS. You can do without the last line in the code above if you can change your database URL to start with postgis.

I got this error when trying to run tests with the test db set like so:

if 'test' in sys.argv:
    DATABASES = {
        'default': {
            'ENGINE': 'django.db.backends.sqlite3', 
            'NAME': '_testdb',
        }
    }

The problem is that the sqlite3 DatabaseOperations object doesn’t have the attribute geo_db_type (like the title of this post suggests).

My solution was to change the backend to the sqlite equivalent GIS engine:

        'ENGINE': 'django.contrib.gis.db.backends.spatialite'

See the Django docs on GeoDjango installation for all the possible backends, with installation instructions: https://docs.djangoproject.com/en/3.0/ref/contrib/gis/install/#spatial-database

I was having the same problem and I had to change:

'ENGINE': 'django.db.backends.postgresql_psycopg2',

to:

'ENGINE': 'django.contrib.gis.db.backends.postgis',

for me helped

1) add 'django.contrib.gis', to INSTALLED_APPS
2) change from

DATABASES = {
'default': {
    'ENGINE': 'django.db.backends.mysql',

to

DATABASES = {
'default': {
    'ENGINE': 'django.contrib.gis.db.backends.mysql', 

This post is old but I just wanted to share my answer to this problem. I’m using the Dj Database package, and I didn’t know that the connection URL is different when using PostGIS. The connection string for PostGIS is postgis://USER:PASSWORD@HOST:PORT/NAME

Hope this helps someone.

I’m using the Python sample application on stack cedar 14 and the regular Heroku buildpack Heroku/python with PostGIS and had the same problem that my database settings were overwritten with the wrong DB engine, which caused heroku run python manage.py migrate to fail with the above error.
Just adding the engine in the settings would not change anything.
After some investigation I found out that it is the call to django_heroku.settings(locals()) in the last line of my settings.py which is reverting my changes.

I fixed it by overwriting the engine again like this by adding a line afterwards:

django_heroku.settings(locals())
DATABASES['default']['ENGINE'] = 'django.contrib.gis.db.backends.postgis'

I changed my default db engine from psycopg2 to postgis

PREVIOUSLY

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        ...,
    }
}

NOW

DATABASES = {
    'default': {
        'ENGINE': 'django.contrib.gis.db.backends.postgis',
        ...,
    }
}

Settings.py > INSTALLED_APPS

INSTALLED_APPS = [
.
.
'django.contrib.gis'
.
]

Settings.py > Databases

DATABASES = {
'default': {
    'ENGINE': 'django.contrib.gis.db.backends.mysql',
     ...,
    }
} 

In may case I was having this issue because I forgot to comment out the db settings further down in settings.py:

# Update database configuration with $DATABASE_URL.
db_from_env = dj_database_url.config(conn_max_age=500)
DATABASES['default'].update(db_from_env)

These lines were overriding the settings that I had added above. Adding this in case someone else had the same issue.