Use a newer image for 3.9 on appveyor

appveyor.yml

@@ -6,6 +6,8 @@ build:
   verbosity: minimal
 # The VS 2019 image doesn't have
 # the MSVC needed for Python 2.7.
+# Note that as of 2020-11-11, this does not include
+# a Python 3.9 build.
 image: Visual Studio 2015
 
 environment:
@@ -27,6 +29,13 @@ environment:
 
   matrix:
     # http://www.appveyor.com/docs/installed-software#python
+
+    - PYTHON: "C:\\Python39-x64"
+      PYTHON_ARCH: "64"
+      PYTHON_VERSION: "3.9.x"
+      PYTHON_EXE: python
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+
     - PYTHON: "C:\\Python27"
       PYTHON_ARCH: "32"
       PYTHON_VERSION: "2.7.x"
@@ -77,15 +86,6 @@ environment:
       PYTHON_VERSION: "3.8.x"
       PYTHON_EXE: python
 
-    - PYTHON: "C:\\Python39"
-      PYTHON_ARCH: "32"
-      PYTHON_VERSION: "3.9.x"
-      PYTHON_EXE: python
-
-    - PYTHON: "C:\\Python39-x64"
-      PYTHON_ARCH: "64"
-      PYTHON_VERSION: "3.9.x"
-      PYTHON_EXE: python
 
 cache:
   - "%TMP%\\py\\"