[issue40979] typing module docs: keep text, add subsections

[Guido van Rossum]

Change by Guido van Rossum <guido at python.org>:


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pull_requests: +20970
pull_request: https://github.com/python/cpython/pull/21843

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40979>
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