Upgrade aiohttp to 3.1.2

[fabaff]

Description:

Changelog: https://github.com/aio-libs/aiohttp/blob/master/CHANGES.rst#312-2018-04-05

Example entry for configuration.yaml (if applicable):

http:

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.