Fix encoding warnings with PEP 597 enabled

[danyeaw]

This PR fixes Encoding Warnings with PEP 597 warnings enabled and enables the warnings in CI. It goes along with python-poetry/poetry#8893 and python-poetry/poetry-core#685.

[bswck]

The contrib is overall a good idea.
I would only say that having to specify the encoding parameter when "opening" /dev/null seems a bit... nonsensical, i.e. it doesn't reflect the motivation behind PEP 597.

[danyeaw]

@bswck I think the idea is that for every open statement we explicitly declare whether the encoding is "utf-8" or "locale", but I agree that /dev/null probably doesn't need an encoding. Do you think it is an issue that we are explicit even with this one?

[danyeaw]

Thanks @Secrus!

[radoering]

I noticed that there is still one open warning:

home/runner/work/cleo/cleo/src/cleo/io/outputs/stream_output.py:53: EncodingWarning: UTF-8 Mode affects locale.getpreferredencoding(). Consider locale.getencoding() instead.

I'm not sure if we even want UTF-8 Mode to affect it. In any case, I think we should decide between:

• Suppress this warning because locale.getpreferredencoding() is what we want.
• Use getencoding() for Python 3.11 and above.