[docs] [issue35631] Improve typing docs wrt abstract/concrete collection types

[Ivan Levkivskyi]

Ivan Levkivskyi <levkivskyi at gmail.com> added the comment:

I think this can be closed now. Whether to merge doc-fix backport to now security-only 3.6 branch is up to Ned.

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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<https://bugs.python.org/issue35631>
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