app.py

import asyncio
from aiohttp import web
from routers import routers
from aiohttp_session.cookie_storage import EncryptedCookieStorage

import aiohttp_cors


loop = asyncio.get_event_loop()
app = web.Application(loop=loop)
aiohttp_cors.setup(app, defaults={
    'http://localhost:3000': aiohttp_cors.ResourceOptions(
        allow_methods=['GET'],
        expose_headers='*',
        allow_headers='*'
    )
})

for router in routers:
    app.router.add_route(router[0], router[1], router[2], name=router[3])

web.run_app(app)

There is no Cross Origin in your case: all your code is executed on local host.