Replace loop.current_task() with officially supported API

[asvetlov]

loop.current_task() was never a part of asyncio API.
Starting from 3.7 third party loops can use dedicated calls for custom task implementation:
asyncio._register_task, ``asyncio._unregister_task, asyncio._enter_task`, `asyncio._leave_task`.

The API calls are started from underscore to emphasize these very low level position (should be used by libraries like tokio only) but they are a part of of official API.

[fafhrd91]

I don't remember if I put anything into aiohttp. feel free to replace it with proper api calls. I will change tokio loop later

[asvetlov]

@fafhrd91 cool.
Thank you to picking up the issue.
I have no rust/tokio experience.

aiohttp uses this functionality: https://github.com/aio-libs/aiohttp/blob/master/aiohttp/helpers.py#L182-L191

Now I'm adding asyncio.current_task() support for Python 3.7 to avoid PendingDeprecationWarning.

I'll support tokio for a while but please fix the issue in a reasonable time.

[fafhrd91]

Ok

[asvetlov]

Cross-link: aio-libs/aiohttp#2826