Try explicitly setting AppVeyor image to VS2022

AppVeyor can't seem to find a python.exe in the path specified for
Python 3.9 or later, so let's explicitly set the image version just in
case somehow an older image is being used— AppVeyor doesn't say which
image is in use on a build's page…?

appveyor.yml

@@ -1,10 +1,12 @@
-# What Python version is installed where:
-# http://www.appveyor.com/docs/installed-software#python
-
+# Deprecated's package version, sync with setup.py
 version: 1.2.14.{build}
+# Explicitly set base image to use
+image: Visual Studio 2022
 
 environment:
   matrix:
+    # What Python version is installed where:
+    # https://www.appveyor.com/docs/windows-images-software/#python
     - PYTHON: "C:\\Python37"
       TOX_ENV: "py37"