fixing workflow for python 3.7

Author: choldgraf

.github/workflows/tests.yml

@@ -12,6 +12,10 @@ jobs:
     steps:
       - name: Checkout source
         uses: actions/checkout@v2
+      - name: Set up Python 3.7
+        uses: actions/setup-python@v1
+        with:
+          python-version: 3.7
       - name: Build package
         run: |
           pip install wheel