docs: import sys in the captured output example

[Hugohong258]

Add the missing sys import to the capsys example, which writes to sys.stderr.

Validation: copied the original snippet into a standalone test module and confirmed it fails with NameError: name 'sys' is not defined; the updated snippet passes using this checkout. The Sphinx HTML build passes with warnings treated as errors.

[RonnyPfannschmidt]

Should we run this via regendoc

[Hugohong258]

Yes, good point. I've made the example regendoc-managed by adding a # content of test_output.py marker and a pytest -q test_output.py output block. I verified that the new regendoc action reproduces the checked-in output, and the pre-commit hooks and Sphinx docs build pass.

[RonnyPfannschmidt]

thanks, wanna add to authors as well? else we merge tomorrow

[Hugohong258]

Thanks! I’ve added Hugohong258 to AUTHORS in alphabetical order.