Do not test with Python 3.6

.github/workflows/testing.yml

@@ -13,10 +13,10 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-20.04  # Downgrade Ubuntu to 20.04 to fix missing Python 3.6
+    runs-on: ubuntu-22.04
     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:
     - uses: actions/checkout@v3