Python 3.7 support

[gastlich]

Python 3.7 introduced new reserved keywords async and await, which makes frontera unusable on the newest python, because of module called async located under following path:

https://github.com/scrapinghub/frontera/blob/master/frontera/utils/async.py

[sibiryakov]

Thank you @gastlich it should be easy to solve

[sibiryakov]

It should be fixed in latest (v0.8) release. Could you check @gastlich ?

[suzil]

I can confirm this is no longer an issue at least with Travis tests #379