Temporarily pin yarl while aiohttp is broken

Aiohttp had an open requirement specification on yarl which has
released a 1.0 that is backwards incompatible.  Pin to <1.0
until https://github.com/aio-libs/aiohttp/issues/2662 is fixed.

Change-Id: I4e750900501ed92bdbb616f5664f7e8ab7fa99c3
1 file changed