MNT: Enable CI with Python 3.9, drop Python 3.6

Author: kernc

Diff for: .github/workflows/ci.yml

@@ -11,11 +11,11 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.6, 3.7]
+        python-version: [3.7, 3.8]
         include:
-          - python-version: 3.8
+          - python-version: 3.9
             test-type: lint
-          - python-version: 3.8
+          - python-version: 3.9
             test-type: docs
 
     steps: