chore: bump project version from 1.38.0 to 1.39.0 and add checkout step to test workflow

Add actions/checkout@v4 step to the test CI workflow before creating the virtual environment, and update pyproject.toml version to 1.39.0 with corresponding CHANGELOG entry documenting the virtual environment test isolation improvement.

.gitea/workflows/test.yml

@@ -12,6 +12,7 @@ jobs:
       PIP_BREAK_SYSTEM_PACKAGES: 1
     runs-on: ubuntu-latest
     steps:
+    - uses: actions/checkout@v4 
     - name: Create virtual environment
       run: |
         python -m venv venv