[issue46494] Mention typing_extensions in the typing documentation

[miss-islington]

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


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nosy: +miss-islington
nosy_count: 7.0 -> 8.0
pull_requests: +29862
pull_request: https://github.com/python/cpython/pull/31746

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<https://bugs.python.org/issue46494>
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