Deprecate passing bytes to TerminalReporter.write_line

[LuanaPBueno]

This deprecates passing bytes to TerminalReporter.write_line(), following the steps nicoddemus described in pytest-dev/pytest#7323. Bytes are still decoded for now, so nothing breaks until pytest 10. I used Claude to help, and reviewed and tested the changes myself.

Closes #7323

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits.
• Add text like closes #XYZW to the PR description and/or commits.
• If AI agents were used, they are credited in Co-authored-by commit trailers.
• Create a new changelog file in the changelog directory.
• Add yourself to AUTHORS in alphabetical order.

[RonnyPfannschmidt]

please use unformattedwarnings

there's a structural issue with -W error triggering problems with the gc if a warning is reused again and again

[LuanaPBueno]

Thanks! Switched to UnformattedWarning! Can you review for me, please?

[RonnyPfannschmidt]

Thanks