[3.8] bpo-38531: document extend action's added version (GH-16865)#16868

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rhettinger merged 1 commit intopython:3.8from

miss-islington:backport-7414207-3.8

Oct 20, 2019

Commits on Oct 20, 2019

bpo-38531: document extend action's added version (GH-16865)

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