Switch to babel

As the comment in `tox.ini` says:

```
# TODO: These are not necessarily easy to get on Windows. Consider switching
# to pybabel, which is pure Python and can be added as dependency to this environment.
# See also https://github.com/python-babel/babel/issues/1014
allowlist_external = xgettext msgmerge
```

That issue has now been fixed by python-babel/babel#1015.