Fix pn.state.onload type hint

[MarcSkovMadsen]

My editor was complaining about the typehint for pn.state.onload when using an async callable as in

import panel as pn
from asyncio import sleep
pn.extension()

pn.panel("Hello").servable()

async def run():
    for i in range(10):
        print(i, " I sleep")
        await sleep(0.1)
        
pn.state.onload(run)

This PR fixes the issue. See also python/typing#424 for guidance on type annotations for async callables.