Let's put it into an add-on that one can install once and forget about it:
bl_info = {
"name": "Random Render Seed",
"author": "Martynas Ziemys",
"version": (1, 0),
"blender": (2, 80, 0),
"location": "CyclesRenderSettings",
"description": "Random Render seed EVERY TIME before rendering",
"warning": " ",
"wiki_url": " "
"",
"category": "Render",
}
import bpy, random
from bpy.app.handlers import persistent
@persistent
def RandomSeedHandler(scene):
bpy.context.scene.cycles.seed = random.randint(10, 1000000000)
def register():
bpy.app.handlers.render_pre.append(RandomSeedHandler)
def unregister():
bpy.app.handlers.render_pre.remove(RandomSeedHandler)
if __name__ == "__main__":
register()
If this is copied to a new text block and saved with .py extension it can be installed as an add-on.