connect to a DB using psycopg2 without password

Surprisingly, the answer is to not specify a host at all. If you do this,

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

Then psycopg2 will connect using a Unix socket in the same manner as psql. When you specify a HOST, psycopg2 will connect with TCP/IP, which requires a password.