Update pytest workflow to make it pass

The order of execution mangled things up. The package installation
actually downgrades some packages (e.g. pylint) which made the
pylint stage fail.

.github/workflows/pytest.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.10"]
+        python-version: ["3.10", "3.9"]
     steps:
       - uses: actions/checkout@v3
       - name: Set up Python ${{ matrix.python-version }}
@@ -18,8 +18,8 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -r tests/requirements.txt
           pip install -e .
+          pip install -r tests/requirements.txt
           pip install coverage pytest
       - name: Execute pytests to avoid regressions
         run: |