Supervisor is not available. Cannot install it on a fresh Debian 9.13

On our vagrant dev environment, when provisioning (Vagrant box debian 9.13) I install Supervisor using shell provisioner. Always working as expected. That's the content of the script:

#!/usr/bin/env bash

sudo DEBIAN_FRONTEND=noninteractive apt-get -y install supervisor=3.*

cp /home/vagrant/projectX/vagrant/configs/supervisor/supervisord.conf /etc/supervisor

chmod 777 -R /etc/supervisor/conf.d

A few days ago, it turns out that Supervisor is not being installed anymore when provisioning.

The error we get is:

Reading package lists… Done
Building dependency tree
Reading state information… Done
Selected version '3.3.5-1' (Debian:10.10/oldstable [all]) for 'supervisor'
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
supervisor : Depends: python-meld3 but it is not installable
E: Unable to correct problems, you have held broken packages.

Also, trying to install manually using:
(Supervisor 4.* is not using python-meld3, that's why tried)

sudo apt-get update
sudo apt-get install supervisor -y

I got:

Reading package lists… Done
Building dependency tree
Reading state information… Done
Package supervisor is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'supervisor' has no installation candidate

Things I have tried:

Installing python-meld3:

sudo apt-get install python-meld3 -y

Reading package lists… Done
Building dependency tree
Reading state information… Done
Package python-meld3 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'python-meld3' has no installation candidate

apt-cache policy python-meld3

python-meld3:
  Installed: (none)
  Candidate: (none)
  Version table:
     1.0.2-2 -10
        -10 http://ftp.us.debian.org/debian stretch/main amd64 Packages
        -10 http://ftp.us.debian.org/debian buster/main amd64 Packages

Also by downloading the code from meld3 Github and installing by hand with same result.

Fix dependencies

sudo apt-get -f install

Reading package lists… Done
Building dependency tree
Reading state information… Done
The following packages were automatically installed and are no longer required:
sgml-base xml-core
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

Nothing supervisor related.

Add repos to sources.list

From here: Link

apt-cache policy supervisor

Shows:

supervisor:
Installed: (none)
Candidate: (none)
Version table:

3.3.5-1 -10

   -10 http://ftp.us.debian.org/debian buster/main amd64 Packages

3.3.1-1+deb9u1 -10

   -10 http://ftp.us.debian.org/debian stretch/main amd64 Packages
   -10 http://security.debian.org/debian-security stretch/updates/main amd64 Packages

Then added to /etc/apt/sources.list and sudo apt-get update and all of them are ignored…

UPDATE: Current sources.list

deb http://ftp.us.debian.org/debian/ stretch main
deb-src http://ftp.us.debian.org/debian/ stretch main

deb http://security.debian.org/debian-security stretch/updates main
deb-src http://security.debian.org/debian-security stretch/updates main

# stretch-updates, previously known as 'volatile'
deb http://ftp.us.debian.org/debian/ stretch-updates main
deb-src http://ftp.us.debian.org/debian/ stretch-updates main

What is happening? What I'm missing? It's being quite difficult to install it. Any help would be really appreciated!

PD: I cannot upgrade to Debian 10 or 11. We are fixed to 9.13