Use boto3 session instead of boto3 client

boto3 client has known issues for multithreading. boto/boto3#801
One solution is to use session instead.

src/django_iam_dbauth/aws/database_wrapper.py

@@ -9,7 +9,8 @@ def get_aws_connection_params(params):
     enabled = params.pop("use_iam_auth", None)
     if enabled:
         region_name = params.pop("region_name", None)
-        rds_client = boto3.client(service_name="rds", region_name=region_name)
+        session = boto3.session.Session()
+        rds_client = session.client(service_name="rds", region_name=region_name)
 
         hostname = params.get("host")
         if hostname: