gh-105713: Document that tokenize raises when mixing tabs/spaces (#105723)

* gh-105713: Document that tokenize raises when mixing tabs/spaces
* Update Doc/whatsnew/3.12.rst

Co-authored-by: Alex Waygood <[email protected]>

Author: lysnikolaou

Doc/whatsnew/3.12.rst

@@ -1499,6 +1499,9 @@ Changes in the Python API
   * Some incomplete or invalid Python code now raises :exc:`tokenize.TokenError` instead of
     returning arbitrary ``ERRORTOKEN`` tokens when tokenizing it.
 
+  * Mixing tabs and spaces as indentation in the same file is not supported anymore and will
+    raise a :exc:`TabError`.
+
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