Add sourcemaps (#3872)

* Update build scripts to produce source maps

package/python.js

@@ -0,0 +1,14 @@
+/**
+ * @license
+ * Copyright 2020 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Python Generator module.
+ */
+
+/* eslint-disable */
+'use strict';
+
+Blockly.Python = BlocklyPython;