chore: use powertools layer

Author: Tom McCarthy

backend/layers/requirements.txt

@@ -1,4 +1,3 @@
 requests==2.22.0
 cognitojwt==1.1.0
-aws-lambda-powertools==1.0.0
 boto3==1.10.34

backend/shoppingcart-service.yaml

@@ -25,6 +25,8 @@ Globals:
     Tracing: Active
     AutoPublishAlias: live
     Runtime: python3.8
+    Layers:
+      - !Sub arn:aws:lambda:${AWS::Region}:017000801446:layer:AWSLambdaPowertoolsPython:3
     Environment:
       Variables:
         TABLE_NAME: !Ref DynamoDBShoppingCartTable