Use socket.sendall to send packets

If only some of the data was transmitted, writelines
does not attempt to send the remaining data, while
sendall continues to send data until either all data
has been sent or an error occurs.

dmoj/packet.py

@@ -110,7 +110,6 @@ def _connect(self):
 
         log.info('Starting handshake with: [%s]:%s', self.host, self.port)
         self.input = self.conn.makefile('rb')
-        self.output = self.conn.makefile('wb', 0)
         self.handshake(problems, versions, self.name, self.key)
         log.info('Judge "%s" online: [%s]:%s', self.name, self.host, self.port)
 
@@ -238,7 +237,8 @@ def _send_packet(self, packet: dict):
         raw = zlib.compress(utf8bytes(json.dumps(packet)))
         with self._lock:
             try:
-                self.output.writelines((PacketManager.SIZE_PACK.pack(len(raw)), raw))
+                assert self.conn is not None
+                self.conn.sendall(PacketManager.SIZE_PACK.pack(len(raw)) + raw)
             except Exception:  # connection reset by peer
                 log.exception('Exception while sending packet to site, will not attempt to reconnect! Quitting judge.')
                 os._exit(1)