Make it more obvious which Python version runs on a given Linux distribution (#15909)

Make it more obvious which Python version runs on a given Linux distribution so when we end up dropping support for a given Python version, we can more easily find the reference to the Python version and remove any references for the distribution. We don't want to be running tests or building packages on a distribution that no longer has a supported Python version.

This way, we can avoid another situation like when we dropped support for Python 3.7 but forgot to drop the Debian Buster references everywhere (https://github.com/matrix-org/synapse/pull/15893)

changelog.d/15909.misc

@@ -0,0 +1 @@
+Document which Python version runs on a given Linux distribution so we can more easily clean up later.

scripts-dev/build_debian_packages.py

@@ -22,14 +22,18 @@ from typing import Collection, Optional, Sequence, Set
 
 # These are expanded inside the dockerfile to be a fully qualified image name.
 # e.g. docker.io/library/debian:bullseye
+#
+# If an EOL is forced by a Python version and we're dropping support for it, make sure
+# to remove references to the distibution across Synapse (search for "bullseye" for
+# example)
 DISTS = (
-    "debian:bullseye",
-    "debian:bookworm",
-    "debian:sid",
-    "ubuntu:focal",  # 20.04 LTS (our EOL forced by Py38 on 2024-10-14)
-    "ubuntu:jammy",  # 22.04 LTS (EOL 2027-04)
-    "ubuntu:kinetic",  # 22.10 (EOL 2023-07-20)
-    "ubuntu:lunar",  # 23.04 (EOL 2024-01)
+    "debian:bullseye",  # (EOL ~2024-07) (our EOL forced by Python 3.9 is 2025-10-05)
+    "debian:bookworm",  # (EOL not specified yet) (our EOL forced by Python 3.11 is 2027-10-24)
+    "debian:sid",  # (EOL not specified yet) (our EOL forced by Python 3.11 is 2027-10-24)
+    "ubuntu:focal",  # 20.04 LTS (EOL 2025-04) (our EOL forced by Python 3.8 is 2024-10-14)
+    "ubuntu:jammy",  # 22.04 LTS (EOL 2027-04) (our EOL forced by Python 3.10 is 2026-10-04)
+    "ubuntu:kinetic",  # 22.10 (EOL 2023-07-20) (our EOL forced by Python 3.10 is 2026-10-04)
+    "ubuntu:lunar",  # 23.04 (EOL 2024-01) (our EOL forced by Python 3.11 is 2027-10-24)
 )
 
 DESC = """\