[docs] [issue25296] Simple End-of-life guide covering all unsupported versions

[Nick Coghlan]

Nick Coghlan added the comment:

Items to capture for each unsupported or security fix only release series:

* date & version of last binary release (maintenance -> security fix only transition)
* date & version of last source-only security release (commercial redistributors are on their own for ongoing support from this point)
* link to the release PEP
* link to the upgrade guide in the next What's New doc once the next version is released

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25296>
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