Missing accept header in http resquests leads to "HTTP Error 406: Not Acceptable"

When using the tar source plugin for this file: https://alioth.debian.org/frs/download.php/file/3979/chrpath-0.16.tar.gz

It fails with this error: Error mirroring https://alioth.debian.org/frs/download.php/file/3979/chrpath-0.16.tar.gz: HTTP Error 406: Not Acceptable

The reason is that buildstream uses urllib.request.urlretrieve from python, and this does not set the Accept header. The accept header needs to be set to /.