fix asyncio worker handling

[benoitc]

simpler code to handle asyncio installation just test the version in the module.

fix #830

[matrixise]

where is the raise?

[tilgovi]

Looks good after you fix the missing raise that @matrixise pointed out.

[tilgovi]

Needs rebase.

[berkerpeksag]

This won't be raised in Python 2 because of the yield from usages. Here's an alternative patch: https://gist.github.com/berkerpeksag/7a5091a7b7944dc9e9c2

With the new patch:

>>> from gunicorn.workers import gaiohttp
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "gunicorn/workers/gaiohttp.py", line 13, in <module>
    raise RuntimeError("You need Python >= 3.3 to use the asyncio worker")
RuntimeError: You need Python >= 3.3 to use the asyncio worker

With #831:

>>> from gunicorn.workers import gaiohttp
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "gunicorn/workers/gaiohttp.py", line 79
    yield from self.wsgi.close()
             ^
SyntaxError: invalid syntax

I can open a new pull request if this approach is good enough.

[benoitc]

sounds can you open a PR with your patch? Will test it in the day :)

[berkerpeksag]

Done: #846.

[benoitc]

related changes in master

[tilgovi]

I deleted the branch. I hope that's okay, @benoitc