title: TooManyRequestsException for Boto3 Client Organization
body: <p>I am fetch all child account from the Master AWS Account by boto3 Organization. 
  Code is working fine. I am able to get child account list. 
  But if you run my AWS Lambda function again then it fail to get Child Accounts.</p>
  
  <p>Getting following error:</p>
  
  <pre><code>Error while getting AWS Accounts : An error occurred (TooManyRequestsException) when calling the ListAccounts operation: AWS Organizations can't complete your request because another request is already in progress. Try again later
  </code></pre>
  
  <p>After 20 to 30 minutes, I can see my code work for once and again raise above exception.</p>
  
  <p>I am Run this code by AWS Gateway + AWS Lambda.</p>
  
  <p>Any idea?</p>
  
  <p>Code:</p>
  
  <pre><code>import boto3
  class Organizations(object):
      """AWS Organization"""
      def __init__(self, access_key, secret_access_key, session_token=None):
          self.client = boto3.client('organizations',
                                     aws_access_key_id=access_key,
                                     aws_secret_access_key=secret_access_key,
                                     aws_session_token=session_token
                                    )
  
      def get_accounts(self, next_token=None, max_results=None):
          """Get Accounts List"""
          if next_token and max_results:
              result = self.client.list_accounts(NextToken=next_token,
                                                 MaxResults=max_results)
          elif next_token:
              result = self.client.list_accounts(NextToken=next_token)
          elif max_results:
              result = self.client.list_accounts(MaxResults=max_results)
          else:
              result = self.client.list_accounts()
  
          return result
  
  class AWSAccounts(object):
      """ Return AWS Accounts information. """    
      def get_aws_accounts(self, access_key, secret_access_key, session_token):
          """ Return List of AWS account Details."""
          org_obj = Organizations(access_key=access_key,
                                  secret_access_key=secret_access_key,
                                  session_token=session_token)
  
          aws_accounts = []
          next_token = None
          next_result = None
          while True:
              response = org_obj.get_accounts(next_token, next_result)
              for account in response['Accounts']:
                  account_details = {"name": account["Name"],
                                     "id": account["Id"],
                                     "admin_role_name": self.account_role_name
                                    }
                  aws_accounts.append(account_details)
  
              if "NextToken" not in response:
                  break
              next_token = response["NextToken"]
  
          return aws_accounts
  </code></pre>