chore: bump project version to 1.39.0 and switch test execution to venv

- Update version string in setup.cfg from 1.38.0 to 1.39.0
- Replace direct pytest invocation with venv-based test runner in CI config
- Add release notes for version 1.39.0 to CHANGELOG.md

.gitea/workflows/test.yml

@@ -18,12 +18,12 @@ jobs:
         . venv/bin/activate
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
-        pip install -e ".[dev]"
+        venv/bin/python3 -m pip install --upgrade pip
+        venv/bin/python3 -m pip install -e ".[dev]"
 
     - name: Run tests with pytest
       run: |
-        pytest --cov=pr --cov-report=xml --cov-report=term-missing
+        venv/bin/pytest --cov=pr --cov-report=xml --cov-report=term-missing
 
     - name: Upload coverage to Codecov
       uses: codecov/codecov-action@v4