Use a newer VM image on Appveyor

So that fewer Pythons need to be installed manually!

https://ci.appveyor.com/project/mgedmin/zodbbrowser/settings/environment
showed I was using the Visual Studio 2015 image by default!

appveyor.yml

@@ -1,9 +1,13 @@
 version: build-{build}-{branch}
 
+# https://www.appveyor.com/docs/lang/python/
+image:
+  - Visual Studio 2019
+
 environment:
   matrix:
-    # https://www.appveyor.com/docs/installed-software#python lists available
-    # versions
+    # https://www.appveyor.com/docs/windows-images-software/#python lists
+    # available versions
     - PYTHON: "C:\\Python27-x64"
     - PYTHON: "C:\\Python37-x64"
     - PYTHON: "C:\\Python38-x64"