Serial communication between python and arduino
If the python Serial write function sends byte, then you should read it in Arduino with Serial.read().

void loop() {
  if (Serial.available()){
    byte data = Serial.read();
    myservo.write(data);
  }
}


If python sends the number as text, then you can read it with parseInt(). 

void loop() {
  if (Serial.available()){
    int data = Serial.parseInt();
    myservo.write(data);
  }
}


The parseInt is one of the blocking functions and will by default wait one second for next char after the last sent digit was received. But the number is sent at once from python so the delays between digits are small. Set in setup() Serial.setTimeout(10);