CI: Drop Python 3.6 tests

Author: effigies

.github/workflows/tests.yml

@@ -27,7 +27,7 @@ jobs:
     strategy:
       matrix:
         os: ['ubuntu-latest']
-        python-version: [3.6, 3.7, 3.8, 3.9, "3.10"]
+        python-version: [3.7, 3.8, 3.9, "3.10"]
         check: ['test']
         pip-flags: ['']
         depends: ['REQUIREMENTS']