Drop Python 3.6 and add 3.8 & 3.9 to AppVeyor

Author: marcus7070

appveyor.yml

@@ -5,10 +5,12 @@ platform:
 
 environment:
     matrix:
-        - PYTHON_VERSION: 3.6
-          MINICONDA_DIRNAME: C:\Miniconda36-x64
         - PYTHON_VERSION: 3.7
           MINICONDA_DIRNAME: C:\Miniconda37-x64
+        - PYTHON_VERSION: 3.8
+          MINICONDA_DIRNAME: C:\Miniconda38-x64
+        - PYTHON_VERSION: 3.9
+          MINICONDA_DIRNAME: C:\Miniconda39-x64
 
     ANACONDA_TOKEN:
         secure: $(anaconda_token)
@@ -27,4 +29,4 @@ test_script:
     - pytest -v --cov
 
 on_success:
-    - codecov
+    - codecov