bpo-33253: Fix xxsubtype.bench to accept correct string signature

[scarletcafe]

This fixes the issue as explained on the tracker where xxsubtype.bench can never run without raising an exception due to the old Python 2 string signature still being in use.

Old behavior:

New behavior:

https://bugs.python.org/issue33253

[the-knights-who-say-ni]

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[miss-islington]

Thanks @Gorialis for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.6, 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-6440 is a backport of this pull request to the 3.7 branch.

[bedevere-bot]

GH-6441 is a backport of this pull request to the 3.6 branch.

[serhiy-storchaka]

Thank you for your contribution @Gorialis!

Signing the CLA is not required for such trivial change. But it would be better to sign it for the case if you will contribute less trivial changes in future.