code: function zipRequirements() {
    const rootZip = new JSZip();
    const src = path.join('.serverless', 'requirements');
    const runtimepath = 'python';
  
    return addTree(rootZip.folder(runtimepath), src).then(() =>
      writeZip(rootZip, path.join('.serverless', 'pythonRequirements.zip'))
    );
  }
docs: Zip up requirements to be used as layer package.
  @return {Promise} the JSZip object constructed.