System test failures cleaning up backups

[c24t]

instance.list_backups fails at this line:

python-spanner/tests/system/test_system.py

Line 125 in 2b74f9c

for backup in instance.list_backups():

because instance is the protobuf type google.cloud.spanner_admin_instance_v1.types.spanner_instance_admin.Instance instead of google.cloud.spanner_v1.instance.Instance. This is causing the following error in CI:

During handling of the above exception, another exception occurred:

    def setUpModule():
        if USE_EMULATOR:
            from google.auth.credentials import AnonymousCredentials

            emulator_project = os.getenv("GCLOUD_PROJECT", "emulator-test-project")
            Config.CLIENT = Client(
                project=emulator_project, credentials=AnonymousCredentials()
            )
        else:
            Config.CLIENT = Client()
        retry = RetryErrors(exceptions.ServiceUnavailable)

        configs = list(retry(Config.CLIENT.list_instance_configs)())

        instances = retry(_list_instances)()
        EXISTING_INSTANCES[:] = instances

        # Delete test instances that are older than an hour.
        cutoff = int(time.time()) - 1 * 60 * 60
        for instance in Config.CLIENT.list_instances("labels.python-spanner-systests:true"):
            if "created" not in instance.labels:
                continue
            create_time = int(instance.labels["created"])
            if create_time > cutoff:
                continue
            # Instance cannot be deleted while backups exist.
>           for backup in instance.list_backups():

tests/system/test_system.py:125:

See e.g. this failing test run.

It looks like it was introduced in #195, but the tests passed for that PR. The API changes to list_instances and list_backups that would have caused this happened back in #147 (see https://github.com/googleapis/python-spanner/pull/147/files#diff-f9e7537fc73135ee5d350541c1147e8ce8c71c505b01c9ea1187f9ee80540b19R328-R356), so I'm not sure why we're only seeing this issue now.