[issue37979] Document an alternative to ISO 8601 parsing

[miss-islington]

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


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pull_requests: +15273
pull_request: https://github.com/python/cpython/pull/15597

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37979>
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