In waybackpy/wrapper.py set self.timestamp to None on init.

In older interface(2.x.x) we had timestamp set to none in the constructer, so maybe it should be best to set it to None in the backwards compatiblliy module.)

waybackpy/wrapper.py

@@ -23,6 +23,7 @@ class Url:
         self.url = url
         self.user_agent = str(user_agent)
         self.archive_url = None
+        self.timestamp = None
         self.wayback_machine_availability_api = WaybackMachineAvailabilityAPI(
             self.url, user_agent=self.user_agent
         )