Specify an image for appveyor that works for Python 2.7

appveyor.yml

@@ -1,9 +1,29 @@
+clone_depth: 50
+max_jobs: 8
+shallow_clone: true
+build:
+  parallel: true
+  verbosity: minimal
+# The VS 2019 image doesn't have
+# the MSVC needed for Python 2.7.
+image: Visual Studio 2015
+
 environment:
   global:
+    APPVEYOR_SAVE_CACHE_ON_ERROR: "true"
     # SDK v7.0 MSVC Express 2008's SetEnv.cmd script will fail if the
-    # /E:ON and /V:ON options are not enabled in the batch script intepreter
+    # /E:ON and /V:ON options are not enabled in the batch script interpreter
     # See: http://stackoverflow.com/a/13751649/163740
     CMD_IN_ENV: "cmd /E:ON /V:ON /C .\\appveyor\\run_with_env.cmd"
+    # Use a fixed hash seed for reproducability
+    PYTHONHASHSEED: 8675309
+    # Don't get warnings about Python 2 support being deprecated. We
+    # know.
+    PIP_NO_PYTHON_VERSION_WARNING: 1
+    PIP_UPGRADE_STRATEGY: eager
+    # Enable this if debugging a resource leak. Otherwise
+    # it slows things down.
+    # PYTHONTRACEMALLOC: 10
 
   matrix:
     # http://www.appveyor.com/docs/installed-software#python
@@ -55,13 +75,21 @@ environment:
       PYTHON_ARCH: "64"
       PYTHON_VERSION: "3.9.x"
 
+cache:
+  - "%TMP%\\py\\"
+  - '%LOCALAPPDATA%\pip\Cache -> appveyor.yml,setup.py'
+
 install:
   - "SET PATH=%PYTHON_ROOT%;%PYTHON_ROOT%\\Scripts;%PATH%"
   - "python --version"
   - "python -c \"import struct; print(struct.calcsize('P') * 8)\""
 
-  # Upgrade pip to the latest version
-  #- "python appveyor\\run_pip.py install --disable-pip-version-check --upgrade pip"
+  # Upgrade to the latest version of pip to avoid it displaying warnings
+  # about it being out of date. Do this here instead of above in
+  # powershell because the annoying 'DEPRECATION:blahblahblah 2.7 blahblahblah'
+  # breaks powershell.
+  - "%CMD_IN_ENV% %PYEXE% -mensurepip -U --user"
+  - "%CMD_IN_ENV% %PYEXE% -mpip install -U --user pip"
 
   # Install requirements for running tests and building artifacts
   - "%CMD_IN_ENV% pip install --upgrade -r dev-requirements.txt"