Drop support of Python 3.3

[kxepal]

aiohttp did that long time ago (last supported release is 0.17.4), so we have to do that as well.

[hugovk]

Here's the pip installs for aiocouchdb from PyPI for the last year -- no 3.3!

$ pypinfo -d 365 --percent --pip aiocouchdb pyversion
python_version percent download_count
-------------- ------- --------------
3.5              94.8%          7,109
3.6               4.1%            309
2.7               0.7%             54
3.4               0.4%             28
2.6               0.0%              1

[kxepal]

Nice! Thank you, @hugovk , for good news!