ci: use pipenv to run shell

.github/workflows/python-test.yml

@@ -26,12 +26,12 @@ jobs:
           pipenv install --dev --deploy
       - name: Lint with black and flake
         run: |
-          black --check icalevents
-          flake8 . --count --exit-zero
+          pipenv run black --check icalevents
+          pipenv run flake8 . --count --exit-zero
       - name: Test with pytest
         run: |
-          coverage run test.py
-          coverage xml
+          pipenv run coverage run test.py
+          pipenv run coverage xml
       - name: 'Upload coverage to Codecov'
         uses: codecov/codecov-action@v2
         with: