test: use python3 instead of python

On some platform, like macOS, python is no longer available by default.

Author: lpinca

test/parallel/test-child-process-set-blocking.js

@@ -25,7 +25,7 @@ const assert = require('assert');
 const ch = require('child_process');
 
 const SIZE = 100000;
-const python = process.env.PYTHON || 'python';
+const python = process.env.PYTHON || 'python3';
 
 const cp = ch.spawn(python, ['-c', `print(${SIZE} * "C")`], {
   stdio: 'inherit'