The ci.yml updates add a commit to PRs. I tested this locally using `act` as best I could, but ran into an issue that I think will resolve when running on the real runner (undefined head). We'll see how it works when the workflows actually run ...

The new workflow adds a badge to the readme based on coverage for Python 3.12.

(for now at least..)

- Switch to using pytest-django to run the Django tests, as that has JUnit support. Add Django settings as a flag rather than in pyproject.toml because defining it there makes the normal pytest run fail since it can't find the module.
- Adds a simple script using junitparser to combine the two JUnit XML files.

```
File read successfully "/home/runner/work/python-edtf/python-edtf/./combined_junit_pytest.xml"
Warning: Your comment is too long (maximum is 65536 characters), coverage report will not be added.
Warning: Try add: "--cov-report=term-missing:skip-covered", or add "hide-report: true", or add "report-only-changed-files: true", or switch to "multiple-files" mode
```