gh-97725: Fix default file in Task.print_stack

[megabug]

asyncio.Task.print_stack is described by documentation as having a default output file of sys.stderr. However, the default None is passed all the way to print statements, leading to an actual default of sys.stdout.

Fix by setting the file to sys.stderr when the default is used.

• Issue: asyncio.Task.print_stack doesn't use sys.stderr by default #97725

[bedevere-bot]

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[ghost]

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[kumaraditya303]

Please add a regression test for this.

[gvanrossum]

But do we want to fix the code to match the docs? In cases like this, where the docs and the code have always disagreed (or for many releases), we tend to prefer to fix the docs to match the code, since "fixing" the code may actually break apps that were expecting the observed behavior.

Also, print_* kind of suggests that it does the same thing as print(). More indication to me that the docs were just wrong (it happens :-).

[kumaraditya303]

Superseded by #101652