title: Update attribute "timestamp" reserved word
body: <p>I need to update the timestamp attribute in my dynamodb table using boto3 but the attribute name "timestamp" is a reserved word so it's throwing an error on the SET command.</p>
  
  <pre class="lang-py prettyprint-override"><code>table.update_item(
      Key={
          'id': item_id
      },
      UpdateExpression='SET timestamp = :val1', # this is the line giving the problem
      ExpressionAttributeValues={
          ":val1": new_timestamp
      }
  )
  </code></pre>
  
  <p>"errorMessage": "An error occurred (ValidationException) when calling the UpdateItem operation: Invalid UpdateExpression: Attribute name is a reserved keyword; reserved keyword: timestamp",</p>