aiohttp examples should use async context manager for session creation

[asvetlov]

E.g. instead of

with aiohttp.ClientSession() as session:
    ...

Use

async with aiohttp.ClientSession() as session:
    ...

Also please be consistent: either pass loop everywhere (aiohttp.Timeout(5, loop=loop), asyncio.gather(*tasks, loop=loop)) or don't pass it at all.

For first case calling asyncio.set_event_loop(None) at very begin of the program effectively checks that loop is passed to any call where it's required.

[vstinner]

Hi Andrew! Would you mind to send a pull request please?

[asvetlov]

Sure, but the question is: do you prefer explicit loop or not?
In all my code I pass loop explicitly because I've found it as best practice for writing safe code and avoiding stupid mistakes.
On other hand it may be too complex for beginners.

[ludovic-gasc]

For me, ok for explicit loop, most AsyncIO libraries do that.

[vstinner]

I shut down the project: #33