docs/python: Add CPython update schedule

doc/languages-frameworks/python.section.md

@@ -1632,3 +1632,25 @@ would be:
 ```ShellSession
 $ maintainers/scripts/update-python-libraries --target minor --commit --use-pkgs-prefix pkgs/development/python-modules/**/default.nix
 ```
+
+## CPython Update Schedule
+
+With [PEP 602](https://www.python.org/dev/peps/pep-0602/), CPython now
+follows a yearly release cadence. In nixpkgs, all supported interpreters
+are made available, but only the most recent two
+interpreters package sets are built; this is a compromise between being
+the latest interpreter, and what the majority of the Python packages support.
+
+New CPython interpreters are released in October. Generally, it takes some
+time for the majority of active Python projects to support the latest stable
+interpreter. To help ease the migration for Nixpkgs users
+between Python interpreters the schedule below will be used:
+
+| When | Event |
+| --- | --- |
+| After YY.11 Release | Bump CPython package set window. The latest and previous latest stable should now be built. |
+| After YY.05 Release | Bump default CPython interpreter to latest stable. |
+
+In practice, this means that the Python community will have had a stable interpreter
+for ~2 months before attempting to update the package set. And this will
+allow for ~7 months for Python applications to support the latest interpreter.