Remove 3.6 from workflow tests

3.6 isn't supported by Ubuntu 22.04

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
     steps:
       - name: Check out code
         uses: actions/checkout@v2