PEP 3147: Update figures

[shallow-beach]

There was a little bit of missing background in the png for the figure, went in and fixed that. To do so, I had to get Dia running on MacOS 15 which was a nightmare (booting the app via XQuartz terminal 🥴), so also rendered out the dia file into an svg for ease of future contributors.

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📚 Documentation preview 📚: https://pep-previews--4178.org.readthedocs.build/

[ghost]

All commit authors signed the Contributor License Agreement.

[hugovk]

Thanks!