bootstrap: add explicit UTF-8 encoding to text-mode open() calls

[ozankenangungor]

This makes all text-mode open() calls in bootstrap.py
explicitly use encoding="utf-8".

Binary-mode ("rb" / "wb") opens are left unchanged.

The change is purely mechanical and does not alter behavior.

Tested with:
./x.py test bootstrap

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[Kobzol]

If the script sets r, it should already be assumed to be text, this is only a concern when there was no "r"/"rb" specified, no?

[apiraino]

And please @ozankenangungor don't invoke Copilot in this project.

[ozankenangungor]

Apologies, I won’t invoke Copilot again in this repository.

Regarding the change: my understanding was that the issue aimed to avoid relying on the platform/locale-dependent default encoding, even in text mode. For that reason, I added encoding="utf-8" to existing text-mode open() calls and left binary opens unchanged.

If this is broader than intended, I’m happy to narrow the scope to only the cases where no mode/encoding was specified.