docs(CHANGES) Note dropping of Python 3.9

Author: tony

CHANGES

@@ -15,6 +15,12 @@ $ pip install --user --upgrade --pre libtmux
 
 - _Future release notes will be placed here_
 
+### Breaking changes
+
+- Drop support for Python 3.9; the new minimum is Python 3.10 (#602). See also:
+  - [Python 3.9 EOL timeline](https://devguide.python.org/versions/#:~:text=Release%20manager-,3.9,-PEP%20596)
+  - [PEP 596](https://peps.python.org/pep-0596/)
+
 ### Development
 
 - Add Python 3.14 to test matrix (#601)