pin pip to 20.2 for now (#4694)

* pin pip to 20.2 for now

* add self to list of trigger files

.github/workflows/pytest.yml

     - 'gym-unity/**'
     - 'test_constraints*.txt'
     - 'test_requirements.txt'
+    - '.github/workflows/pytest.yml'
   push:
     branches: [master]
 
       run: python -c "import sys; print(sys.version)"
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
+        # pin pip to workaround https://github.com/pypa/pip/issues/9180
+        python -m pip install pip==20.2
         python -m pip install --upgrade setuptools
         python -m pip install --progress-bar=off -e ./ml-agents-envs -c ${{ matrix.pip_constraints }}
         python -m pip install --progress-bar=off -e ./ml-agents -c ${{ matrix.pip_constraints }}