Fix extraction order

Fix extraction order

Before Babel 2.3, the input directories were always extracted
in the order specified by the command line arguments.

For example, `pybabel extract dir_a dir_b dir_c` would process
`dir_a` first, then `dir_b`, and finally `dir_c`.

Since Babel 2.3, the inputs are stored in a Python dictionary,
and we loop over items in the dictionary using items().
This makes the order unspecified, not matching the order
specified on the command line anymore.