Fix AppVeyor build
Windows cannot replace a running executable. It's not clear why this was
working until now.
@@ -50,7 +50,7 @@ install:
# Upgrade to the latest version of pip to avoid it displaying warnings
# about it being out of date.
- - pip install --disable-pip-version-check --user --upgrade pip wheel
+ - python -m pip install --upgrade pip wheel
- pip install tox virtualenv
test_script:
