Update pytest command

.github/workflows/ci.yml

@@ -12,13 +12,14 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Install Python
+      - name: Install Python 3
         uses: actions/setup-python@v1
         with:
-          python-version: 3.8
+          python-version: 3.6
       - name: Install poetry
         run: |
           python -m pip install --upgrade pip
           pip install poetry
+          poetry install
       - name: Run tests with pytest
         run: poetry run pytest