Python script not reading from serial unless teensy is rebooted
Serial communication is always terminated with a specific character (usually \n). The receiving device will accumulate the incoming bytes in a buffer until it receives this terminating character.

To fix the problem simply change the last line of your arduino loop to Serial.println(sensor);

Also note that the arduino might be faster than your python script, meaning that you will get several sensor values per readout. If that's the case you have to add a short delay in the arduino loop.