[issue20813] Backport revised 2to3 guide to older branches

[Nick Coghlan]

Nick Coghlan added the comment:

We should add the new content in issue 20812 before doing the backport.

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dependencies: +Explicitly cover application migration in the 2->3 guide

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20813>
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