Bump aiohttp contraint to <3.5.0
Bump aiohttp contraint to <3.5.0

New version contains many bugfixes, a couple new features and seemingly no breaking changes (for this lib's purposes at least).

Relevant changes: https://github.com/aio-libs/aiohttp/blob/master/CHANGES.rst#340-2018-08-25
Harmon758/discord.py,imayhaveborkedit/discord.py,Rapptz/discord.py,rapptz/discord.py,Harmon758/discord.py,khazhyk/discord.py