Add SystemChannelFlags.join_notification_replies

Add SUPPRESS_JOIN_NOTIFICATION_REPLIES system channel flag
See: discord/discord-api-docs#3977

discord/flags.py

@@ -216,6 +216,15 @@ class SystemChannelFlags(BaseFlags):
         """
         return 4
 
+    @flag_value
+    def join_notification_replies(self):
+        """:class:`bool`: Returns ``True`` if sticker reply button ("Wave to say hi!") is
+        shown for member join notifications.
+
+        .. versionadded:: 2.0
+        """
+        return 8
+
 
 @fill_with_flags()
 class MessageFlags(BaseFlags):