Specify Python v3.6 (#5)

Author: biglovisa

.github/workflows/publish.yml

@@ -14,7 +14,7 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v2
         with:
-          python-version: '3.x'
+          python-version: '3.6'
 
       - name: Install dependencies
         run: make build

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v2
         with:
-          python-version: '3.x'
+          python-version: '3.6'
       - name: Run tests
         env:
           SANDBOX_API_KEY: ${{ secrets.SANDBOX_API_KEY }}