django: set SchemaEditor.sql_create_fk = None

It disables parts of tests that don't apply until support for
foriegn keys is added (refs #313).

Author: timgraham

django_spanner/schema.py

@@ -10,6 +10,7 @@
 class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     sql_create_table = "CREATE TABLE %(table)s (%(definition)s) PRIMARY KEY(%(primary_key)s)"
     sql_delete_table = "DROP TABLE %(table)s"
+    sql_create_fk = None
     # Spanner doesn't support partial indexes. This string omits the
     # %(condition)s placeholder so that partial indexes are ignored.
     sql_create_index = "CREATE INDEX %(name)s ON %(table)s%(using)s (%(columns)s)%(extra)s"