bpo-35689: IDLE: Add docstrings and unittests for colorizer.py (GH-11472)

(cherry picked from commit ee0f927bd8dba805a04963dbec1ad49fe830b842)

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
diff --git a/Lib/idlelib/NEWS.txt b/Lib/idlelib/NEWS.txt
index 815d8aa..65662d9 100644
--- a/Lib/idlelib/NEWS.txt
+++ b/Lib/idlelib/NEWS.txt
@@ -3,6 +3,8 @@
 ======================================
 
 
+bpo-35689: Add docstrings and tests for colorizer.
+
 bpo-35833: Revise IDLE doc for control codes sent to Shell.
 Add a code example block.