Fix GitHub Actions workflow for Python tests by specifying PYTHONPATH

.github/workflows/pytests.yaml

@@ -7,32 +7,40 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
           python-version: 3.8
+
       - name: Install Poetry
         uses: snok/install-poetry@v1
         with:
           virtualenvs-create: true
           virtualenvs-in-project: true
+
       - name: Load cached venv
         id: cached-poetry-dependencies
         uses: actions/cache@v2
         with:
           path: .venv
           key: venv-${{ runner.os }}-${{ hashFiles('**/poetry.lock') }}
+
       - name: Install dependencies
         if: steps.cached-poetry-dependencies.outputs.cache-hit != 'true'
         run: poetry install --no-interaction --no-root
+
       - name: Run tests
         run: |
           source .venv/bin/activate
+          export PYTHONPATH=$PYTHONPATH:$(pwd)
           pytest tests/
+
       - name: Run black
         run: |
           source .venv/bin/activate
           black --check --diff .
+
       - name: Run flake8
         run: |
           source .venv/bin/activate