[skip changelog] Temporarily full pin Python in integration test CI workflow (#1442)

A bug introduced in the 3.9.7 release of Python causes a spurious failure of the
`test\test_lib.py::test_install_git_invalid_library` integration test:
https://bugs.python.org/issue45121

As a workaround, the Python version used by the "Test Integration" CI workflow that runs the integration tests is pinned
to the last working version: 3.9.6. Since it is convenient to get automatic updates for Python patch releases, this full
pin should be reverted back to the "3.9" minor version pin once a new version of Python is released with the bug fixed
and added to versions available for installation via the `actions/setup-python` GitHub Actions action.

.github/workflows/test-go-integration-task.yml

@@ -5,7 +5,7 @@ env:
   # See: https://github.com/actions/setup-go/tree/v2#readme
   GO_VERSION: "1.16"
   # See: https://github.com/actions/setup-python/tree/v2#available-versions-of-python
-  PYTHON_VERSION: "3.9"
+  PYTHON_VERSION: "3.9.6"
 
 # See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
 on: