Babel configuration

[lafrech]

Since the changes made for v4, particularly moving to pyproject.toml, the babel config is broken. I removed all makefiles.

I'm afraid things are not as simple as adding a few lines in pyproject.toml. It might require custom commands / entry points, as there doesn't seem to be a replacement to setuptools integration.

Currently, things can be achieved manually using the pybabel command and tweaking files manually, but that's not ideal.

TODO:

• Figure out a way to make things as straightforward as possible
• Document

[lafrech]

I couldn't find any simple way. Manual editing is required.

I added a few hints to RELEASING.rst for now.