CCL-467: install the package in CI so packaging tests are not skipped

The hand-written packaging tests arriving with the platform-side sync read the
built distribution's metadata through importlib.metadata and skip when no
distribution is installed, which is exactly the regression they guard against
(every release up to 4.28.0 published requires_python: None). Installing the
package also makes --cov measure the installed artifact.

Author: centml-machine-user

.github/workflows/python.yml

@@ -38,6 +38,9 @@ jobs:
           python -m pip install --upgrade pip
           pip install -r requirements.txt
           pip install -r test-requirements.txt
+          # install the package itself: the packaging tests read the built distribution's metadata
+          # through importlib.metadata and skip silently when it is absent
+          pip install .
       - name: Test with pytest
         run: |
           pytest --cov=platform_api_python_client