If you were only using PySerial, then there would be some logic in connecting TXD to "RTS". The idle state of a TTL UART when not transmitting data happens to be logic high. Toggling to low and back, relatively slowly, is called a serial break and can be done easily with PySerial's sendBreak() method. That would have the advantage that you only need permissions for the serial port, not for raw GPIO.
However, if you are using RPi.GPIO already, you may as well connect any other GPIO pin to "RTS". The pin does not understand true serial signalling. It is just a logic input that puts the device into low power mode when driven low.
(There is a third option, too. GPIO17 on the Pi can be configured as RTS. That normally tells the remote device whether there is room in the input buffer to receive characters, but it can also be driven as an arbitrary logic signal using PySerial's setRTS() method. This was obviously the intention and why they labelled the power/sleep input as "RTS" in the first place. But it is not a good choice on a Pi because RTS is disabled by default, and neither RPi.GPIO nor PySerial include a method to configure it on GPIO17.)