Appveyor: Convert Python install script to PowerShell

Also install Python only if not already on wanted version

scripts/win-update-python.ps1

@@ -0,0 +1,13 @@
+$CurrentVersion = python -c "import sys; print('%s.%s.%s' % sys.version_info[0:3])"
+Write-Output "Python installed: $CurrentVersion, wanted: $env:PYTHON_VERSION"
+
+if ($CurrentVersion -ne $env:PYTHON_VERSION) {
+  $InstallerUrl = 'https://www.python.org/ftp/python/' + $env:PYTHON_VERSION + '/python-' + $env:PYTHON_VERSION + '-amd64.exe'
+  Write-Output "Downloading and installing $InstallerUrl..."
+  (new-object net.webclient).DownloadFile($InstallerUrl, 'python-amd64.exe')
+  python-amd64.exe /quiet InstallAllUsers=1 TargetDir=%PYTHON% Include_doc=0 Include_tcltk=0 Include_test=0
+  python -m ensurepip
+}
+
+python --version
+pip3 --version