chg: Add more python versions

.github/workflows/nosetests.yml

@@ -6,17 +6,22 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.7, 3.8, 3.9]
 
     steps:
-    - uses: actions/checkout@v1
-    - name: Set up Python 3.7
-      uses: actions/setup-python@v1
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{matrix.python-version}}
+      uses: actions/setup-python@v2
       with:
-        python-version: 3.7
+        python-version: ${{matrix.python-version}}
+
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip poetry
         poetry install
+
     - name: Test with nosetests
       run: |
         poetry run mypy .