build: Require python 3.5

contrib/devtools/README.md

@@ -167,7 +167,7 @@ still compatible with the minimum supported Linux distribution versions.
 
 Example usage after a gitian build:
 
-    find ../gitian-builder/build -type f -executable | xargs python contrib/devtools/symbol-check.py 
+    find ../gitian-builder/build -type f -executable | xargs python3 contrib/devtools/symbol-check.py
 
 If only supported symbols are used the return value will be 0 and the output will be empty.