[FLINK-14288][legal] Add Py4J NOTICE for source release

This closes #9816.
diff --git a/NOTICE b/NOTICE
index 88121c1..4b38233 100644
--- a/NOTICE
+++ b/NOTICE
@@ -33,6 +33,7 @@
 See bundled license files for details.
 
 - d3:3.5.12
+- net.sf.py4j:py4j:0.10.8.1
 
 This project bundles the following dependencies under SIL OFL 1.1 license (https://opensource.org/licenses/OFL-1.1).
 See bundled license files for details.