Currently PyMySQL determines SSL/TLS verify_mode automatically here. https://github.com/PyMySQL/PyMySQL/blob/master/pymysql/connections.py#L337

Related issues: (it is not a PyMySQL bug.) #430

My suggestion is to add configuration so that users can specify any of the verify_mode (ssl.CERT_NONE, ssl.CERT_OPTIONAL, ssl.CERT_REQUIRED) defined in ssl.py.
https://docs.python.org/3/library/ssl.html

It would be good to support ssl-mode explained here.
https://dev.mysql.com/doc/refman/5.7/en/connection-options.html#option_general_ssl-mode