[3.12] gh-105713: Document that tokenize raises when mixing tabs/spaces (GH-105723) (#105725)

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

(cherry picked from commit ed8217b)

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

Author: 3 people

Doc/whatsnew/3.12.rst

@@ -1498,6 +1498,9 @@ 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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