Retry new_data_cb() if data isn't available upstream

Using the already-used-elsewhere backoff library. Retry every 10 minutes
for a maximum of two hours.

Author: Arto Jantunen

custom_components/nordpool/__init__.py

@@ -1,9 +1,9 @@
 import logging
 from collections import defaultdict
-from datetime import datetime, timedelta
-from functools import partial
+from datetime import timedelta
 from random import randint
 
+import backoff
 import voluptuous as vol
 from homeassistant.config_entries import ConfigEntry
 from homeassistant.core import Config, HomeAssistant
@@ -143,6 +143,10 @@ async def new_hr(_):
             _LOGGER.debug("Called new_hr callback")
             async_dispatcher_send(hass, EVENT_NEW_HOUR)
 
+        @backoff.on_exception(
+            backoff.constant,
+            (InvalidValueException),
+            logger=_LOGGER, interval=600, max_time=7200, jitter=None)
         async def new_data_cb(_):
             """Callback to fetch new data for tomorrows prices at 1300ish CET
             and notify any sensors, about the new data