[issue26063] Update copyright in the devguide

[Roundup Robot]

Roundup Robot added the comment:

New changeset a4c18139d9b0 by Ezio Melotti in branch 'default':
#26063: future-proof copyright updating.
https://hg.python.org/devguide/rev/a4c18139d9b0

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<http://bugs.python.org/issue26063>
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