Merge pull request #93 from zopefoundation/cleaner-releases

Avoid spurious "failures" on Appveyor during releases

appveyor.yml

@@ -22,6 +22,7 @@ install:
   - pip install -e .[test,ZODB]
 
 build_script:
+  - python -m pip install -U pip
   - pip install wheel
   - python -W ignore setup.py -q bdist_wheel
 
@@ -33,6 +34,6 @@ artifacts:
     name: wheel
 
 deploy_script:
-  - ps: if ($env:APPVEYOR_REPO_TAG -eq $TRUE) { pip install twine; twine upload dist/* }
+  - ps: if ($env:APPVEYOR_REPO_TAG -eq $TRUE) { pip install twine; twine upload --skip-existing dist/* }
 
 deploy: on