client.py

import aiohttp
import asyncio
from multidict import MultiDict

async def main():

    headers = MultiDict({"User-Agent": "ua1"})
    headers.extend({"User-Agent": "ua2"})

    async with aiohttp.ClientSession(headers=headers) as session:
        async with session.get('http://localhost:8000') as response:

            text = await response.text()
            print("Request sent.")

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

server.py

#!/usr/bin/env python3

import http.server as SimpleHTTPServer
import socketserver as SocketServer
import logging

PORT = 8000

class GetHandler(
        SimpleHTTPServer.SimpleHTTPRequestHandler
        ):

    def do_GET(self):
        logging.error(self.headers)
        SimpleHTTPServer.SimpleHTTPRequestHandler.do_GET(self)


Handler = GetHandler
httpd = SocketServer.TCPServer(("", PORT), Handler)

httpd.serve_forever()