Update caldav to 2.1.0

This requires adding vobject to the caldav component's requirements
because the caldav package no longer depends on it:
python-caldav/caldav#527

Author: dotlambda

homeassistant/components/caldav/manifest.json

@@ -6,5 +6,5 @@
   "documentation": "https://www.home-assistant.io/integrations/caldav",
   "iot_class": "cloud_polling",
   "loggers": ["caldav", "vobject"],
-  "requirements": ["caldav==1.6.0", "icalendar==6.3.1"]
+  "requirements": ["caldav==2.1.0", "icalendar==6.3.1", "vobject==0.9.9"]
 }