Run with Python UTF-8 mode enabled?

There's [a proposal](https://discuss.python.org/t/pep-597-enable-utf-8-mode-by-default-on-windows/3122) for Python to use UTF-8 mode on Windows by default. Most significantly, this means that `open()`-ing a file in text mode would use UTF-8 by default rather than a locale-dependent encoding.

This is controversial, and one step that has been suggested is for Jupyter to enable UTF-8 mode for kernels before a change is made in Python, to explore how well it works. Changing the default is arguably also more favourable to new programmers who don't have a lot of existing code, and Jupyter is often used in teaching settings.

I'm strongly in favour of changing the default in Python, but I'd be -0.5 on only changing it in Jupyter, because that would mean code inside Jupyter behaves differently from code outside. But I'd be more in favour if changing it in Jupyter was a prelude to, or test case for, changing it in Python.

Start with the linked PEP 597 proposal and the discussion in this issue. Trace how ipykernel launches kernels and how Python's open() default encoding is affected on Windows. Done means a project decision, backed by evidence about enabling UTF-8 mode in Jupyter.

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