Specific gast==0.3.3 (#26278)

python/requirements.txt

@@ -2,7 +2,7 @@ requests>=2.20.0
 numpy>=1.12, <=1.16.4 ; python_version<"3.5"
 numpy>=1.12 ; python_version>="3.5"
 protobuf>=3.1.0
-gast>=0.3.3
+gast==0.3.3
 matplotlib<=2.2.4 ; python_version<"3.6"
 scipy>=0.19.0, <=1.2.1 ; python_version<"3.5"
 nltk>=3.2.2, <=3.4 ; python_version<"3.5"