Update CI on AppVeyor: use oldest and latest supported Python

We do not want to test all Python versions on AppVeyor
because it takes too much time, but testing oldest and latest
is at least better than testing two old versions.
For this we need to change the base image.

.appveyor.yml

@@ -1,21 +1,21 @@
 # This file is part of BenchExec, a framework for reliable benchmarking:
 # https://github.com/sosy-lab/benchexec
 #
-# SPDX-FileCopyrightText: 2007-2020 Dirk Beyer <https://www.sosy-lab.org>
+# SPDX-FileCopyrightText: 2007-2024 Dirk Beyer <https://www.sosy-lab.org>
 #
 # SPDX-License-Identifier: Apache-2.0
 
+image: "Visual Studio 2022"
+
 environment:
   matrix:
     - PYTHON: "C:\\Python38"
-    - PYTHON: "C:\\Python39"
+    - PYTHON: "C:\\Python312"
 
 build: off
 
 install:
   - set PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%
-  # The package installation via our setup.py file uses easy_install, which fails to correctly install lxml.
-  # As some Python environments don't have lxml preinstalled, install it here to avoid errors during the execution in those cases.
   - python -m pip install --user ".[dev]"
 
 test_script: