bpo-45644 Make json.tool soak up infile before writing to outfile

[CharString]

So that

$ python -m json.tool foo.json foo.json

doens't result in an empty foo.json.

https://bugs.python.org/issue45644

[the-knights-who-say-ni]

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[CharString]

Failed tests and changes on main branch instead of feature branch. Sorry for the mess. I couldn't find a way to change the source branch of the PR