test: use python3 instead of python in pummel test

As f9bfe78 already did for a regular test, replace `python` with
`ptyhon3` in the only `pummel` test spawning it so that it can be run on
platforms that don't provide a `python` binary anymore, like modern
macOS or some Linux distributions (e.g. Fedora) without specifying a
`PYTHON` env var.

test/pummel/test-child-process-spawn-loop.js

@@ -24,7 +24,7 @@ const common = require('../common');
 const assert = require('assert');
 
 const spawn = require('child_process').spawn;
-const python = process.env.PYTHON || 'python';
+const python = process.env.PYTHON || (common.isWindows ? 'python' : 'python3');
 
 const SIZE = 1000 * 1024;
 const N = 40;