After the Odoo package is built, the package.py script uses a Docker
image to test the package. After that python3-qrcode was added as a
dependency for the package in 2ff49c5f , it revealed some issues.

A missing cli parameter and a forgotten '&&' in the Dockerfile prevented
the installation of the depending packages.

Also, the script shebang was wrong by launching python2.

Finally, the fact that python3-xlwt is missing in Debian stretch was
highlighted. With this commit, the python3-xlwt is explicitly removed
from the dependencies and the documentation is updated accordingly.

closes odoo/odoo#28807