bpo-40360: [doc] Rephrase deprecation note about lib2to3 (GH-28122)

Author: ambv

Doc/library/2to3.rst

@@ -464,12 +464,15 @@ and off individually. They are described here in more detail.
 
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-.. deprecated:: 3.10
- Python 3.9 will switch to a PEG parser (see :pep:`617`), and Python 3.10 may
- include new language syntax that is not parsable by lib2to3's LL(1) parser.
- The ``lib2to3`` module may be removed from the standard library in a future
- Python version. Consider third-party alternatives such as `LibCST`_ or
- `parso`_.
+.. deprecated-removed:: 3.11 3.13
+ Python 3.9 switched to a PEG parser (see :pep:`617`) while lib2to3 is
+ using a less flexible LL(1) parser. Python 3.10 includes new language
+ syntax that is not parsable by lib2to3's LL(1) parser (see :pep:`634`).
+ The ``lib2to3`` module was marked pending for deprecation in Python 3.9
+ (raising :exc:`PendingDeprecationWarning` on import) and fully deprecated
+ in Python 3.11 (raising :exc:`DeprecationWarning`).
+ It will be removed from the standard library in Python 3.13.
+ Consider third-party alternatives such as `LibCST`_ or `parso`_.
 
 .. note::