Return annotation of response.json()

[jonathanslenders]

Right now, the function signature of response.json() returns Any.

The issue with this is that mypy won't complain when we write:

data = await response.json()

The result is not awaitable. This is common however, when migrating from other HTTP clients like aiohttp.

Ideally, I'd like the return type to be something like this:

-> Union[str, int, dict, list]

I'm not sure whether that's actually feasible.

[tomchristie]

We'd want -> Union[str, int, float, bool, dict, list], right?

Switching to that would probably be okay. It's not complete (because we're not fully expressing the types of dict and list*), but it is sufficient to catch this kind of error.

* Note that Python can't fully express the type that .json() returns.