Debian unstable chroot creation fails

[xolox]

While researching xolox/python-executor#10 I found that the following code fails:

program = AptMirrorUpdater(
    distributor_id='debian',
    distribution_codename='testing',
)
program.create_chroot('/tmp/debian-testing')

Here are the relevant error messages:

2018-10-07 15:42:55 peter-mbp apt_mirror_updater[3668] INFO Updating package lists of chroot (/tmp/debian-unstable) ..
Hit:1 http://ftp.nl.debian.org/debian unstable InRelease
Ign:2 http://ftp.nl.debian.org/debian unstable-updates InRelease
Err:3 http://ftp.nl.debian.org/debian unstable-updates Release                 
  404  Not Found [IP: 130.89.149.21 80]
Ign:4 http://security.debian.org unstable/updates InRelease                    
Err:5 http://security.debian.org unstable/updates Release                      
  404  Not Found [IP: 151.101.36.204 80]
Get:6 http://ftp.nl.debian.org/debian unstable/main Translation-en [6211 kB]
Get:7 http://ftp.nl.debian.org/debian unstable/contrib amd64 Packages [60.8 kB]
Get:8 http://ftp.nl.debian.org/debian unstable/contrib Translation-en [50.7 kB]
Get:9 http://ftp.nl.debian.org/debian unstable/non-free amd64 Packages [96.4 kB]
Get:10 http://ftp.nl.debian.org/debian unstable/non-free Translation-en [94.3 kB]
Reading package lists... Done                                        
E: The repository 'http://ftp.nl.debian.org/debian unstable-updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://security.debian.org unstable/updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

For now I'll just leave this here, if it bites other people I can investigate further.