[3.6] bpo-31728: Prevent crashes in _elementtree due to unsafe cleanup of Element.text and Element.tail (GH-3924)

[miss-islington]

(cherry picked from commit 39ecb9c)

https://bugs.python.org/issue31728

[the-knights-who-say-ni]

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[miss-islington]

@orenmn and @serhiy-storchaka: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @serhiy-storchaka!