retry on 429 throttled status (plotly#1307)

* retry on 429 throttled status
* Added comment describing main source of 429 errors

Author: michaelbabyn

Diff for: plotly/api/v2/utils.py

@@ -114,8 +114,8 @@ def get_headers():
 def should_retry(exception):
     if isinstance(exception, exceptions.PlotlyRequestError):
         if (isinstance(exception.status_code, int) and
-                500 <= exception.status_code < 600):
-            # Retry on 5XX errors.
+                (500 <= exception.status_code < 600 or exception.status_code == 429)):
+            # Retry on 5XX and 429 (image export throttling) errors.
             return True
         elif 'Uh oh, an error occurred' in exception.message:
             return True