Fix cast doing I/O in event loop

[OttoWinter]

Description:

Follow-up on #12474.

We're doing I/O inside async_setup_platform, because the pychromecast.Chromecast constructor calls some http endpoint on the chromecast. We shouldn't do that. I've now looked through the code again, and have not found another instance of this.

Thanks to @rytilahti for pointing this out!

Though this should fix the issue, we could also additionally wrap the whole call in a timeout, to make it even more secure: (Doesn't work since pychromecast.Chromecast isn't a coroutine)

with async_timeout.timeout(30, loop=hass.loop):
    chromecast = yield from hass.async_add_job(
        pychromecast.Chromecast, *want_host)

Example entry for configuration.yaml (if applicable):

media_player:
  - platform: cast

Checklist:

• The code change is tested and works locally.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works. (Dunno how we can check whether a mock object was called from within the event loop)

[balloob]

Ugh for constructors doing work