Pin python version 3.9 into the horreum-base

[lampajr]

Followup of #2080.

Pin the python version that we embed in the Horreum container, this comes with some good improvements:

• Reproducibility of the build
• Clearly state the python version being used
• Ensure CI uses and test the same version we use in the container

Changes proposed

• Pin the python version that is bundled in the Horreum container
• Choose version 3.9 as this is the one installed in registry.access.redhat.com/ubi9/openjdk-17 by default
• Align CI to use the same version, i.e., 3.9

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.