_is_closed check doesn't work with PooledMySQLDatabase and pymysql
Nice, I tested this out and it appears to work correctly. Thanks!

We encountered the same problem as described here: https://github.com/coleifer/peewee/issues/783

Our connections in the pool were closed by MySQL with the 2006 error after reaching server-side timeout. Because we don't always control the environment we run in, we didn't want to guess the my.cnf wait_timeout value and hardcode it in the `stale_timeout` argument for Pool, we just always need to restart a dead connection.

But the solution was simple. The main problem lies [here](https://github.com/coleifer/peewee/blob/7857f08a6fb70fc91c8e884d9eaa2289155d29f5/playhouse/pool.py#L168). This check doesn't work, because `pymysql.connection` also has the `open` attribute (at least in our version 0.6.6). We could use some other method to differentiate between  pymysql and MySQLdb, but there is actually no need. Even though `conn.ping` always [returns None](https://github.com/farcepest/MySQLdb1/blob/d236bb7f6b45f81f0156c4a0cfdef2319ba825d9/_mysql.c#L2011) on MySQLdb, it still does the right thing: throws exception on a dead connection.

After this fix PooledMySQLDatabase recognizes closed connections and everything works good. 
