Before this patch, the debian package depends on `python-pybabel`.
According to the documentation, this is a dummy package for transition
from `python-pybabel` to `python-babel`[1].
This dummy package has thus been removed in debian stretch in favor of
`python-babel`, and the odoo package is thus not installable in debian
stretch.
To fix this, we depend directly on `python-babel`, which is available in
all debian releases[2].
Closes#13905
[1] https://packages.debian.org/jessie/python-pybabel
[2] https://packages.debian.org/jessie/python-babel
097f46e09a