Compatibility with new setuptools

[frenzymadness]

With setuptools 62.1 and newer, it's not possible to build this package. The error says:

error: Multiple top-level packages discovered in a flat-layout: ['po', 'debian'].

To avoid accidental inclusion of unwanted files or directories,
setuptools will not proceed with this build.

If you are trying to create a single distribution with multiple packages
on purpose, you should not rely on automatic discovery.
Instead, consider the following options:

1. set up custom discovery (`find` directive with `include` or `exclude`)
2. use a `src-layout`
3. explicitly set `py_modules` or `packages` with a list of names

To find more information, look for "package discovery" on setuptools docs.

[leigh123linux]

Duplicate #446