17-futures/countries/flags_asyncio.py fails to download flags

[cdm-work]

When running with python 3.6.6, with aiohttp 3.3.2, the 'flags_asyncio.py' script outputs 20 pairs of lines similar to this:
Unclosed client session
client_session: <aiohttp.client.ClientSession object at 0x10ecbfd68>
followed by 20 tracebacks similar to this:
Task exception was never retrieved
future: <Task finished coro=<download_one() done, defined at flags_asyncio.py:28> exception=TypeError("'_SessionRequestContextManager' object is not iterable",)>
Traceback (most recent call last):
File "flags_asyncio.py", line 30, in download_one
image = yield from get_flag(cc) # <7>
File "flags_asyncio.py", line 23, in get_flag
resp = yield from aiohttp.request('GET', url) # <4>
TypeError: '_SessionRequestContextManager' object is not iterable

[cdm-work]

Pull request #26 would address this issue. Obviously it would be out of sync with the book contents, but it would actually work and produce a positive result.