Fix CI compatibility and add Gotify priority/Markdown support

[swayll]

This PR addresses several issues and adds new functionality:

1. CI Fix: The GitHub Actions workflow was failing because it tried to test Django 5.0 with Python 3.9. Django 5.0 requires Python 3.10+. I've updated the workflow matrix to exclude this combination.
2. New Features: Added support for setting Gotify message priority and enabling Markdown content directly from Django's EmailMessage. This is achieved using custom headers (X-Gotify-Priority, X-Gotify-Markdown) or the markdown content subtype.
3. Tests: Added comprehensive tests to verify that the new headers are correctly parsed and passed to the Gotify client.
4. Documentation: Created a CHANGELOG.md for the initial 0.1.0 release.
5. Consistency: Updated comments and docstrings to English.

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PR created automatically by Jules for task 5672123181047253457 started by @swayll

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