Cast activity enumerator to integer instead of accessing value directly

Should make the library more resilient to future changes.

discord/activity.py

@@ -175,7 +175,7 @@ class Activity(_ActivityTag):
                 continue
 
             ret[attr] = value
-        ret['type'] = self.type.value
+        ret['type'] = int(self.type)
         return ret
 
     @property

discord/enums.py

@@ -380,6 +380,9 @@ class ActivityType(Enum):
     listening = 2
     watching = 3
 
+    def __int__(self):
+        return self.value
+
 class HypeSquadHouse(Enum):
     bravery = 1
     brilliance = 2