enable pip cache in appveyor build (#4546)

appveyor.yml

@@ -50,5 +50,8 @@ after_test:
 artifacts:
   - path: dist\*
 
+cache:
+  - '%LOCALAPPDATA%\pip\Cache'
+
 #on_success:
 #  - TODO: upload the content of dist/*.whl to a public wheelhouse