Bump letpot to 0.6.4 (home-assistant#157781)

Author: jpelgrom

homeassistant/components/letpot/manifest.json

@@ -8,5 +8,5 @@
   "iot_class": "cloud_push",
   "loggers": ["letpot"],
   "quality_scale": "silver",
-  "requirements": ["letpot==0.6.3"]
+  "requirements": ["letpot==0.6.4"]
 }