fix: unsubscribing twice from the same pubsub channel raises a KeyError exception

[ilolis]

Trying to unsubscribe twice from the same pubsub channel raises a KeyError exception upon calling get_message().

Steps to reproduce:

import asyncio
import coredis


async def test():
    r = coredis.Redis.from_url("redis://127.0.0.1:6379")
    p = r.pubsub()

    await p.subscribe('channel')
    for _ in range(0, 10):
        await p.get_message(timeout=0.01)

    await p.unsubscribe('channel')
    await asyncio.sleep(0.1)
    await p.get_message()

    # unsub again
    await p.unsubscribe('channel')
    await asyncio.sleep(0.1)

    # the below will throw exception
    await p.get_message()


asyncio.run(test())

[alisaifee]

Thanks! Added a corresponding test case for this scenario here: c856d29