asyncio atexit

Adds atexit functionality to asyncio:

import asyncio_atexit

async def close_db():
    await db_connection.close()

asyncio_atexit.register(close_db)

atexit is part of the standard library, and gives you a way to register functions to call when the interpreter exits.

asyncio doesn't have equivalent functionality to register functions when the event loop exits:

This package adds functionality that can be considered equivalent to atexit.register, but tied to the event loop lifecycle. It:

1. accepts both coroutines and synchronous functions
2. should be called from a running event loop
3. calls registered cleanup functions when the event loop closes
4. only works if the application running the event loop calls close()