Fix deprecation warnings for invalid escape characters

What does the merge request do?

See title, found by @t0b3. I noticed two more instances of this in the recent DXF improvements.

It looks like the branches for 1.2.x and main have not diverged yet, so no need to backport afaics?

Summary for release notes

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Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history