s/isfile/exists/ since Python doesn't regard a socket as a 'file'

Python doesn't regard a proper socket as a file, so I'd have to use exists here instead of isfile. 

I also removed looking at '/tmp/mysql.sock' (although this is the default path on BSD / Mac OS X) because of the possibility of /tmp being world-writable and lacking a socket created by MySQL with proper permissions, which could possibly open up applications using CyMySQL to a man-in-the-middle attack.

Apologies for the minor oversight. :)