Convert PLATFORMS to str type to fix schema validation

[zacharee]

Fixes #33

[JohNan]

Thanks, but I don't think the configuration options has any effect. It looks like I have forgot to remove some kind of template code. The entire configuration can probably be removed.

In any case, I suggest to cast it to a str on line 179 in config_flow instead, because the constant is used elsewhere.

[zacharee]

I checked the other usages, and the Home Assistant API accepts str anywhere it accepts Platform. I'm guessing that's for backwards compatibility.