CI: pin the version of pathlib to v2.3.6 (#28832)

.github/workflows/unit_tests.yaml

@@ -127,6 +127,10 @@ jobs:
           if python -c 'import sys; sys.exit(not sys.version_info >= (3, 6))'; then
               pip install --upgrade flake8 isort>=4.3.5 mypy>=0.900 black
           fi
+    - name: Pin pathlib for Python 2.7
+      if: ${{ matrix.python-version == 2.7 }}
+      run: |
+          pip install -U pathlib2==2.3.6
     - name: Setup git configuration
       run: |
           # Need this for the git tests to succeed.