Fix PY_SSIZE_T_CLEAN warning

Author: edenhill

src/confluent_kafka/src/Producer.c

@@ -228,7 +228,7 @@ Producer_produce0 (Handle *self,
 static PyObject *Producer_produce (Handle *self, PyObject *args,
 				       PyObject *kwargs) {
 	const char *topic, *value = NULL, *key = NULL;
-	int value_len = 0, key_len = 0;
+        Py_ssize_t value_len = 0, key_len = 0;
 	int partition = RD_KAFKA_PARTITION_UA;
 	PyObject *headers = NULL, *dr_cb = NULL, *dr_cb2 = NULL;
         long long timestamp = 0;

src/confluent_kafka/src/confluent_kafka.h

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include <structmember.h>
 #include <pythread.h>