[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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nosy: +miss-islington
nosy_count: 9.0 -> 10.0
pull_requests: +18680
pull_request: https://github.com/python/cpython/pull/19315

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36541>
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