Using a Pi 3 with Raspbian Stretch Lite
Installed Pyserial with: easy_install -U pyserial
Connected Nextion display on Pi UART, Pin 14 TX , Pin 15 RX.
Pi TX (Pin 14) --------> Nextion RX (Mine was yellow wire)
Pi RX (Pin 15) --------> Nextion TX (Mine was blue wire)
Downloaded Nextion Editor, Nextion Editor