[docs] [issue37574] Mention spec_from_loader() in Finder.find_spec() docs.

[miss-islington]

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


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pull_requests: +15526
pull_request: https://github.com/python/cpython/pull/15885

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<https://bugs.python.org/issue37574>
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