Replace codecs.open with built-in open for UTF-8 file handling

[PredaaA]

codecs.open is deprecated since Python 3.14. This replaces all occurences of it to open(...).

[eemeli]

Sorry for not getting to this earlier. It looks like there's a difference in \r\n newline treatment between codecs.open and the builtin open?

[eemeli]

Hey; hope you don't mind that I went ahead and fixed this PR? The test failures were due to needing newline='\n' as an argument for open() so that it doesn't apply universal newlines when reading files.

Beyond that, mocking builtins.open breaks stuff at least in babel, and seems like an in-general bad idea. So I refactored the patch_files() decorator to build actual files instead, using TemporaryDirectory.

[PredaaA]

Thank you for the fixes! I wasn't able to look into it.