[docs] [issue27519] update the references to http://mercurial.selenic.com

[Roundup Robot]

Roundup Robot added the comment:

New changeset 2edae149e8c5 by Berker Peksag in branch 'default':
Issue #27519: Update Mercurial links to mercurial-scm.org
https://hg.python.org/devguide/rev/2edae149e8c5

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nosy: +python-dev

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