Await session closing

[id13]

In order to comply with aiohttp >= 3.0 warning pytest_sanic/utils.py:227: RuntimeWarning: coroutine 'ClientSession.close' was never awaited

I checked this patch for aiohttp >= 3.0 and 2.0 <= aiohttp < 3.0

[coveralls]

Coverage increased (+2.7%) to 97.717% when pulling ede4682 on id13:master into f78925f on yunstanford:master.

[yunstanford]

@id13 Thanks for the PR. I think it's ok to upgrade to aiohttp>=3.0. Can you also add the version specifier in setup.py ? https://github.com/yunstanford/pytest-sanic/blob/master/setup.py#L12

Thanks a lot.

[id13]

@yunstanford done