[ci] force downgrade of yarl for aiohttp tests

[palazzem]

Overview

Some versions of aiohttp may use yarl>=1.0 that drops some methods. In that case it's not possible to run tests for aiohttp because of:

  File "/root/project/.tox/py34-aiohttp12-aiohttp_jinja012/lib/python3.4/site-packages/aiohttp/web_urldispatcher.py", line 14, in <module>
    from yarl import URL, quote, unquote
ImportError: cannot import name 'quote'

Pinning yarl to version 0.18 fixes the problem.

[gabsn]

+1

[delner]

👍