azure: Install x86 python in vs2015 image when building for x86

Use the x86 python when building for x86, to allow tests building python
extension modules to work

ci/azure-steps.yml

@@ -44,10 +44,14 @@ steps:
      }
 
      if ($env:compiler -eq 'msvc2015') {
+       if ($env:arch -eq 'x86') {
+         $forcex86 = "--forcex86"
+       }
+
        # download and install python3 and add to path (since it's not installed in vs2015 image!)
        Set-ExecutionPolicy Bypass -Scope Process -Force
        iex ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))
-       choco install python3 -y --no-progress --params "/InstallDir:C:\Python3"
+       choco install python3 -y --no-progress $forcex86 --params "/InstallDir:C:\Python3"
        $env:Path = "C:\Python3;$env:Path"
 
        # add JDK installed in vs2015 image to PATH