fix CI on py3.5

Author: graingert

.github/workflows/main.yml

@@ -36,7 +36,8 @@ jobs:
         python-version: ${{ matrix.python }}
     - name: Install tox
       run: |
-        python -m pip install --upgrade pip
+        python -m pip install pip-with-requires-python
+        python -m pip install -U pip
         pip install tox
     - name: Test
       run: |