Fix crash on exporting to dxf with superscript characters

What does the merge request do?

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Before the change, there was a crash reported on exporting an SVG containing text with superscript 5 (⁵) to dxf. Reported here: Issue #593

Reproduced on Windows 11.

After the change, the .dxf file is created as expected.

Implementation notes

cp1252 seems to be the default code page for Windows files when unspecified. So, python expects the files it reads to be of this format. This causes an error as the SVG infile is in UTF-8 format. The error is not reproduced on Linux as the system's default encoding is UTF-8.

So, I changed the code to read the file as a UTF-8 file. (inkex/extensions.py file)

Summary for release notes

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A crash (specific to Windows) that occurred on exporting .svg files with superscript characters to .dxf is fixed.

Checklist

• Add unit tests (if applicable) - I'm not quite sure how to do this yet, but will work on it.
• Changes to inkex/ are well documented
• Clean merge request history