Update aiomysql from 0.0.16 to 0.0.20

Author: pyup-bot

requirements-dev.txt

@@ -2,7 +2,7 @@ aiohttp-jinja2==1.1.0
 aiohttp-security==0.4.0
 aiohttp-session==2.7.0
 aiohttp==3.5.4
-aiomysql==0.0.16
+aiomysql==0.0.20
 aiopg==0.14.0
 coverage==4.5.1
 docker==3.3.0