migrations: Use django.db.backends.postgresql.schema.

This replaces django.db.backends.postgresql_psycopg2, which has been
an alias to django.db.backends.postgresql since Django 1.9.

analytics/migrations/0015_clear_duplicate_counts.py

@@ -1,5 +1,5 @@
 from django.db import migrations
-from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
 from django.db.migrations.state import StateApps
 from django.db.models import Count, Sum