Fix test workflows

Pin to tox v3 for now.
(tox plugins aren't ready for tox v4 yet.)

.github/workflows/integration-test.yml

@@ -60,7 +60,7 @@ jobs:
         run: |
           set -x
           python --version
-          pip install tox
+          pip install 'tox<4'
           tox --version
       - name: Test ${{ matrix.config.tox }}
         run: |

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
         uses: actions/checkout@v2
       - name: Install tox-gh-matrix
         run: |
-          python -m pip install tox tox-gh-matrix
+          python -m pip install 'tox<4' 'tox-gh-matrix<0.2'
           python -m tox --version
       - name: Generate tox envlist
         id: generate-envlist