Fix the automatic uploading to the QGIS plugins repository

[ptormene]

Currently we are uploading the zipfile by hand on the QGIS plugins website. In the past the automatic uploading worked, but it has been broken for years.

This might be useful: https://github.com/qgis/QGIS-Django/blob/master/qgis-app/plugins/tests/test_plugin_upload.py