[issue36541] Make lib2to3 grammar better match Python, support the := walrus

[miss-islington]

Change by miss-islington <mariatta.wijaya+miss-islington at gmail.com>:


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pull_requests: +22624
stage: resolved -> patch review
pull_request: https://github.com/python/cpython/pull/23768

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<https://bugs.python.org/issue36541>
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