docs: Add note about Python 3.9 to CHANGELOG.md (#3052)

rules_python 1.5.0 upgraded its internal setuputils to 78.1.1 which has
a minimum supported Python version of 3.9. Using this version with
Python 3.8 leads to errors (see below) although for some reason, I only
see them on Linux builds, not Mac.

Since Python 3.8 is EOL, document that Python 3.8 will no longer work
due to this setuptools version.

Fixes #3050

---------

Co-authored-by: Richard Levasseur <richardlev@gmail.com>

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7b2dfc3..ea76c5a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -109,7 +109,8 @@
 * (py_wheel) py_wheel always creates zip64-capable wheel zips
 * (providers) (experimental) {obj}`PyInfo.venv_symlinks` replaces
   `PyInfo.site_packages_symlinks`
-* (deps) Updating setuptools to patch CVE-2025-47273.
+* (deps) Updated setuptools to 78.1.1 to patch CVE-2025-47273. This effectively makes
+  Python 3.9 the minimum supported version for using `pip_parse`.
 
 {#1-5-0-fixed}
 ### Fixed