conftest change to backport to ch 5, isolation serializable in all pg tests

Author: hjwp

tests/conftest.py

@@ -47,7 +47,7 @@ def wait_for_redis_to_come_up():
 
 @pytest.fixture(scope='session')
 def postgres_db():
-    engine = create_engine(config.get_postgres_uri())
+    engine = create_engine(config.get_postgres_uri(), isolation_level='SERIALIZABLE')
     wait_for_postgres_to_come_up(engine)
     metadata.create_all(engine)
     return engine