ci: specify junit.xml path for Codecov test results upload

The 'Upload test results to Codecov' step was failing with 'No JUnit XML
reports found' because the codecov-action could not auto-discover the file.
The poe 'cover' task generates junit.xml in the project root via
--junitxml=junit.xml, so we explicitly set file: junit.xml.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bearomorphism

.github/workflows/pythonpackage.yml

@@ -68,3 +68,4 @@ jobs:
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
           report_type: test_results
+          file: junit.xml