twine upload --skip-existing

This way if the sdist hasn't been uploaded yet for some reason, Appveyor
will take care of it.

appveyor.yml

@@ -34,6 +34,6 @@ artifacts:
     name: wheel
 
 deploy_script:
-  - ps: if ($env:APPVEYOR_REPO_TAG -eq $TRUE) { pip install twine; twine upload dist/*.whl }
+  - ps: if ($env:APPVEYOR_REPO_TAG -eq $TRUE) { pip install twine; twine upload --skip-existing dist/* }
 
 deploy: on