Mention that code.co_lnotab is deprecated in what's new for 3.10. (#24902)

Author: markshannon

Doc/whatsnew/3.10.rst

@@ -175,6 +175,8 @@ Tracing events, with the correct line number, are generated for all lines of cod
 
 The ``f_lineo`` attribute of frame objects will always contain the expected line number.
 
+The ``co_lnotab`` attribute of code objects is deprecated and will be removed in 3.12.
+Code that needs to convert from offset to line number should use the new ``co_lines()`` method instead.
 
 PEP 634: Structural Pattern Matching
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