Drop unsupported Python versions from CI; add new stable versions.

This drops Python 3.5, 3.6, and 3.7 from the CI workflows, since they
are all EOL. It adds 3.9, 3.10, and 3.11 as new versions we test
against.

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8]
+        python-version: ["3.8", "3.9", "3.10", "3.11"]
 
     steps:
       - uses: actions/checkout@v1