AppVeyor: pin httpbin deps as is done on Unix builds

appveyor.yml

@@ -82,7 +82,7 @@ before_test:
 - ps: |
     Write-Output "Getting and launching httpbin."
     $env:PATH = "C:\Python38;C:\Python38\Scripts;" + $env:PATH
-    pip.exe --disable-pip-version-check install httpbin
+    pip.exe --disable-pip-version-check install Flask==2.0.3 Werkzeug==2.0.3 httpbin
     Start-Job -Name wx_httpbin { python.exe -m httpbin.core 2>&1 > c:\projects\wxwidgets\httpbin.log }
     Start-Sleep -Seconds 5
     curl.exe --silent --show-error http://127.0.0.1:5000/ip > $null