gh-70280: allow using custom sys.displayhook's with doctest

[skirpichev]

This partially reverts patch from https://bugs.python.org/issue8048

Now it's possible to test interactive examples in applications, which use cusomized sys.displayhook, e.g. SymPy, without nasty workarrounds (i.e. overriding the sys.__displayhook__ value before each tests).

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This is a restored PR #25651 with minimal changes (internal for a doctest sys.displayhook changes now aren't leaked out).

• Issue: gh-70280

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