Add the appveyor magic to install Python 3.8

appveyor.yml

@@ -18,6 +18,13 @@ environment:
 
 install:
   - "SET PATH=C:\\Python%PYTHON%;c:\\Python%PYTHON%\\scripts;%PATH%"
+  - ps: |
+      $env:PYTHON = "C:\\Python${env:PYTHON}"
+      if (-not (Test-Path $env:PYTHON)) {
+        curl -o install_python.ps1 https://raw.githubusercontent.com/matthew-brett/multibuild/11a389d78892cf90addac8f69433d5e22bfa422a/install_python.ps1
+        .\install_python.ps1
+      }
+  - ps: if (-not (Test-Path $env:PYTHON)) { throw "No $env:PYTHON" }
   - echo "C:\Program Files\Microsoft SDKs\Windows\v7.1\Bin\SetEnv.cmd" /x64 > "C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\amd64\vcvars64.bat"
   - pip install -e .[test,ZODB]