PyMySQL is a pure Python MySQL client providing a DB-API to a MySQL database by talking directly to the server via the binary client/server protocol.
You can explicitly use pymysql by changing the 'mysql' part of your engine url to 'pymysql'.
You can also make pymysql the default mysql dialect as follows::
import pymysql_sa pymysql_sa.make_default_mysql_dialect()
In this case you don't need to change the engine url.
Being pure Python, PyMySQL is easily patched by gevent and the likes to make it cooperative.