aiocouchdb pip install is downgrading my aiohttp version

[ghost]

when i try to install aiocouchdb its uninstall my aiohttp-2.2.5
and install a lower version.

here is what happened:
pip install aiocouchdb
Collecting aiocouchdb
Downloading aiocouchdb-0.9.1.tar.gz (63kB)
100% |████████████████████████████████| 71kB 523kB/s
Collecting aiohttp==0.17.4 (from aiocouchdb)
Downloading aiohttp-0.17.4.tar.gz (475kB)
100% |████████████████████████████████| 481kB 968kB/s
Requirement already satisfied: chardet in /Users/veto/webs/merkuro/env/lib/python3.6/site-packages (from aiohttp==0.17.4->aiocouchdb)
Installing collected packages: aiohttp, aiocouchdb
Found existing installation: aiohttp 2.2.5
Uninstalling aiohttp-2.2.5:
Successfully uninstalled aiohttp-2.2.5
Running setup.py install for aiohttp ... done
Running setup.py install for aiocouchdb ... done
Successfully installed aiocouchdb-0.9.1 aiohttp-0.17.4

(env)$ pip freeze
aiocouchdb==0.9.1
aiodns==1.1.1
aiohttp==0.17.4