[3.14] gh-137821: Fix wrongly tested json.decoder.scanstring

[corona10]

• Issue: Convert _json to Argument Clinic #137821

[corona10]

@hugovk It was detected from #140780 (comment)

Python 3.11.6 (v3.11.6:8b6ee5ba3b, Oct 2 2023, 11:18:21) [Clang 13.0.0 (clang-1300.0.29.30)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> json.decoder.scanstring(b"xxx", 10)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: first argument must be a string, not bytes