Remove testing of older Python versions

Author: christiansandberg

.github/workflows/pythonpackage.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, '3.x']
+        python-version: ['3.x']
 
     steps:
     - uses: actions/checkout@v2