gh-120952: Add subprocess.shell() function

[vstinner]

• Issue: Add subprocess.shell(cmd) function: similar to os.system(), but use subprocess #120952

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📚 Documentation preview 📚: https://cpython-previews--121093.org.readthedocs.build/

[vstinner]

@gpshead @zooba: Would you mind to review this change?

[zooba]

PR looks fine to me. I'm still not convinced we should add the function at all though.

[vstinner]

I failed to convince enough people, I prefer to abandon this function. Let's keep the status quo, os.system().