Support ssl=True (#700)

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

src/MySQLdb/connections.py

@@ -134,6 +134,8 @@ class Connection(_mysql.connection):
             see the MySQL documentation for more details
             (mysql_ssl_set()).  If this is set, and the client does not
             support SSL, NotSupportedError will be raised.
+            Since mysqlclient 2.2.4, ssl=True is alias of ssl_mode=REQUIRED
+            for better compatibility with PyMySQL and MariaDB.
 
         :param bool local_infile:
             enables LOAD LOCAL INFILE; zero disables