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.

zerver/migrations/0236_remove_illegal_characters_email_full.py

@@ -3,7 +3,7 @@
 from unicodedata import category
 
 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
 
 NAME_INVALID_CHARS = ['*', '`', "\\", '>', '"', '@']