fix xvfb tests (#61)

.github/workflows/test_and_deploy.yml

@@ -104,13 +104,13 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install setuptools tox tox-gh-actions
+          python -m pip install setuptools tox tox-gh-actions
 
       - name: Test with tox
         uses: GabrielBB/xvfb-action@v1
         timeout-minutes: 3
         with:
-          run: tox
+          run: python -m tox
         env:
           PLATFORM: ${{ matrix.platform }}
           BACKEND: ${{ matrix.backend }}