Predictable POT file ordering

Every time an activity maintainer uses the `python setup.py genpot`
command the resulting po/Activity.pot file has changes caused by random
ordering of filesystem inodes.

This makes the output unpredictable.

Sort the list of source files.

src/sugar3/activity/bundlebuilder.py

@@ -583,6 +583,7 @@ def cmd_genpot(config, options):
                 file_path = os.path.relpath(os.path.join(root, file_name),
                                             config.source_dir)
                 python_files.append(file_path)
+    python_files.sort()
 
     # First write out a stub .pot file containing just the translated
     # activity name, then have xgettext merge the rest of the