bump CI to Python 3.7 to match new minimum

Author: qrkourier

.github/workflows/main.yml

@@ -26,7 +26,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-        python-version: '3.6'
+        python-version: '3.7'
 
     - name: Install dependencies
       run: |