As of aiohttp v0.18.0 we dropped support for Python 3.3 up to 3.4.1. The main reason for that is the :meth:`object.__del__` method, which is fully working since Python 3.4.1 and we need it for proper resource closing.
The last Python 3.3, 3.4.0 compatible version of aiohttp is v0.17.4.
This should not be an issue for most aiohttp users (for example Ubuntu 14.04.3 LTS provides python upgraded to 3.4.3), however libraries depending on aiohttp should consider this and either freeze aiohttp version or drop Python 3.3 support as well.