Cleanup some encoding warnings

[danyeaw]

Fixes a few encoding warnings recently introduced in the tests. There is a ruff rule for this called PLW1514, but it is only available if preview rules are enabled.

PR Checklist

Please check if your PR fulfills the following requirements:

• I have read, and I am following the Contributor guide
• I have read, and I understand the GNOME Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bug fix
• Feature
• Chore (refactoring, formatting, local variables, other cleanup)
• Documentation content changes

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[amolenaar]

Is there a reason not to enable the --preview flag?

With this check in place, do we still need PYTHONWARNDEFAULTENCODING=true in our build script(s)?

[danyeaw]

Hi @amolenaar, I was initially turned off to it since it sounds like enabling beta portions of Ruff. But I agree, let's give it a try. I'll update the PR. Thanks!

[danyeaw]

@amolenaar I set preview to true and added PLW1514. Unfortunately, it wasn't detecting even straightforward issues like an open() with no encoding.

[amolenaar]

Then, let's merge this. It's a nice fix for sure.