[3.6] bpo-30737: Update DevGuide links to new URL (GH-3228)

[miss-islington]

Update old devguide links from https://docs.python.org/devguide to https://devguide.python.org
(cherry picked from commit 384899d)

https://bugs.python.org/issue30737

[the-knights-who-say-ni]

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