MySQL use ssl by default but MariaDB don't.
Until mysqlclient<=2.2.1, `ssl=True` unintentionally allowed and it
called `mysql_ssl_set(mysql, NULL, NULL, NULL, NULL, NULL)`. Although it
is no-op in MySQL Connector, MariaDB Connector silently set
MYSQL_OPT_SSL_ENFORCE when the API is called. (See #698)
In case of PyMySQL, ssl is not used by default but `ssl=True` behave
like `sslmode="PREFERRED"`.
For better backward compatibility and compatibility with PyMySQL and
security, I decided to allow ssl=True and it means sslmode="REQUIRED" on
MySQL Connector and
set MYSQL_OPT_SSL_ENFORCE on MariaDB Connector.
Fix#699
MySQL 8.0.33+ shows deprecation warning to stderr.
So we avoid using it as possible.
In the future, we will deprecate `reconnect` option of the
`Connection.ping()`.
