Python: Make import of DataFlowPrivate private

Otherwise you are able to use `DataFlow::isExpressionNode` where
`isExpressionNode` is defined in `DataFlowPrivate.qll`.

Author: RasmusWL

python/ql/src/experimental/dataflow/internal/DataFlowUtil.qll

@@ -2,7 +2,7 @@
  * Contains utility functions for writing data flow queries
  */
 
-import DataFlowPrivate
+private import DataFlowPrivate
 import DataFlowPublic
 
 /**