[debt] EncodingWarning when using pytest's tmpdir.join(...).write(...)

[abravalheri]

In the last CI tests we can see the following warnings:

setuptools/tests/config/test_setupcfg.py: 54 warnings
setuptools/tests/test_bdist_egg.py: 2 warnings
setuptools/tests/test_dist.py: 1 warning
setuptools/tests/test_easy_install.py: 1 warning
setuptools/tests/test_setuptools.py: 2 warnings
setuptools/tests/test_packageindex.py: 1 warning
setuptools/tests/test_virtualenv.py: 1 warning
  /home/runner/work/setuptools/setuptools/.tox/py/lib/python3.11/site-packages/_pytest/_py/error.py:87: EncodingWarning: 'encoding' argument not specified
    return func(*args, **kwargs)

This seems to came from the fact that we are using tmpdir.join(...)write(...).
This method does not allow to specify the encoding argument.

We could replace it with write_text(..., encoding="utf-8").
However the tmpdir fixture itself is deprecated, so another possibility would be moving to tmp_dir.

Part of #3810.

[abravalheri]

Since tmpdir is a legacy API and there is already support for encoding in write_text(), I think it is unlikely this will change upstream.

[Avasam]

LocalPath.open and LocalPath.write_text both support setting an encoding.
The only advantage of write over write_text, is that in non-b mode it'll automatically stringify the data.
https://github.com/pytest-dev/pytest/blob/main/src/_pytest/_py/path.py#L903

So we have two paths forward:

1. Update the fixture (should probably be done anyway)
2. Change write for write_text and add encoding where it's still missing.

Either way, I've added a todo comment in #4255

(note: small typos in issue title: "debit" --> "debt", and missing . before "write")