Update changelog for mixed-indentation removal

Python3 raises:
TabError: inconsistent use of tabs and spaces in indentation

Author: Pierre-Sassoulas

ChangeLog

@@ -36,7 +36,11 @@ Release date: TBA
 
 * bad-continuation and bad-whitespace have been removed, black or another formatter can help you with this better than Pylint
 
-Close #246, #289, #638, #747, #1148, #1179, #1943, #2041, #2301, #2304, #2944, #3565
+  Close #246, #289, #638, #747, #1148, #1179, #1943, #2041, #2301, #2304, #2944, #3565
+
+* mixed-indentation has been removed as python3 will raise a `TabError: inconsistent use of tabs and spaces in indentation` anyway
+
+  Close #2984 #3573
 
 What's New in Pylint 2.5.0?
 ===========================

doc/whatsnew/2.6.rst

@@ -17,3 +17,5 @@ Other Changes
 =============
 
 * `bad-continuation` and `bad-whitespace` have been removed, black or another formatter can help you with this better than Pylint
+
+* `mixed-indentation` has been removed, it became a TabError in python3