Add 3.9 to appveyor file for completeness sake

Are we done with appveyor now?
Can we just remove this file?

.appveyor.yml

diff --git a/.appveyor.yml b/.appveyor.yml
index bcefe02..fbc41c3 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -24,6 +24,8 @@
 #    - PYTHON_VER: Python37-x64
 #    - PYTHON_VER: Python38
 #    - PYTHON_VER: Python38-x64
+#    - PYTHON_VER: Python39
+#    - PYTHON_VER: Python39-x64
 
 #init:
 #- ps: iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))