As suggested by anonymoose's comment and RPi.GPIO's documentation RPi.GPIO module basics setting a pin to output allows to specify an initial value:
To set up a channel as an output:
GPIO.setup(channel, GPIO.OUT)
(where channel is the channel number based on the numbering system you have specified (BOARD or BCM)).
You can also specify an initial value for your output channel:
GPIO.setup(channel, GPIO.OUT, initial=GPIO.HIGH)