Restructure workflow steps

.github/workflows/tests.yml

@@ -29,15 +29,16 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install ruff pytest
           if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
       - name: Lint with Ruff
         if: ${{ matrix.python-version != '3.6' }}
         run: |
+          pip install ruff
           # stop the build if there are Python syntax errors or undefined names
           ruff --output-format=github --select=E9,F63,F7,F82 .
           # default set of ruff rules with GitHub Annotations
           ruff --output-format=github .
       - name: Test with pytest
         run: |
+          pip install pytest
           python -m pytest