Update docgen.yml

Author: narendasan

.github/workflows/docgen.yml

@@ -69,6 +69,7 @@ jobs:
         run: |
           cp toolchains/ci_workspaces/WORKSPACE.x86_64 WORKSPACE
           cd py
+          python3 -m pip install pip==21.3.1
           echo $(which python3)
           echo $(python3 -c 'import site; print(site.getsitepackages()[0])')
           mkdir -p /opt/circleci/.pyenv/versions/3.9.4/lib/python3.9/