Remove test run on Python 3.6 -- it is not available on Ubuntu 22.04.

Author: hodgestar

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, "3.10", "3.11", "3.12", "pypy3.9"]
+        python-version: [3.7, 3.8, 3.9, "3.10", "3.11", "3.12", "pypy3.9"]
         pytest-extra-options: ["--strict-config -W \"ignore:pkg_resources is deprecated\""]
         include:
           - python-version: "pypy2.7"