Fix occasional "endpoint_resolver" failures

It seems that the boto3 library is not threadsafe.  The solution
discussed in GitHub issues such as

  boto/botocore#1246
  boto/boto3#1592

and the documentation at

  https://boto3.amazonaws.com/v1/documentation/api/latest/guide/resources.html?highlight=multithreading#multithreading-multiprocessing

suggest a very simple change that sems to make things work.

Author: mhucka

handprint/services/amazon.py

@@ -82,7 +82,8 @@ def amazon_result(self, file_path, variant, api_method, image_keyword,
         if __debug__: log('setting up Amazon client function "{}"', variant)
         creds = self._credentials
         try:
-            client = boto3.client(variant, region_name = creds['region_name'],
+            session = boto3.session.Session()
+            client = session.client(variant, region_name = creds['region_name'],
                                   aws_access_key_id = creds['aws_access_key_id'],
                                   aws_secret_access_key = creds['aws_secret_access_key'])
             if __debug__: log('calling Amazon API function')