move unit tests from nose tests to pytest

.github/workflows/main.yaml

@@ -11,7 +11,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: '3.x'
+          python-version: [3.6, 3.7, 3.8, 3.9, 3.10]
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip