Update changelog for mixed-indentation removal

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

Author: Pierre-Sassoulas

ChangeLog

@@ -8,11 +8,15 @@ 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
 
 * The no-space-check option has been removed. It's no longer possible to consider empty line like a `trailing-whitespace` by using clever options
 
-Close #1368
+    Close #1368
+
+* mixed-indentation has been removed, it is no longer useful since TabError is included directly in python3
+
+   Close #2984 #3573
 
 What's New in Pylint 2.5.1?
 ===========================

doc/whatsnew/2.6.rst

@@ -19,3 +19,5 @@ Other Changes
 * `bad-continuation` and `bad-whitespace` have been removed. `black` or another formatter can help you with this better than Pylint
 
 * The `no-space-check` option has been removed, it's no longer possible to consider empty line like a `trailing-whitespace` by using clever options.
+
+* `mixed-indentation` has been removed, it is no longer useful since TabError is included directly in python3