chore: some lint fixes [(#3749)](GoogleCloudPlatform/python-docs-samples#3749)

Author: Takashi Matsuo

samples/samples/backup_sample_test.py

@@ -11,11 +11,11 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import random
+import string
 
 from google.cloud import spanner
 import pytest
-import random
-import string
 
 import backup_sample