Upgrade virtualenv

Author: hugovk

.azure-pipelines/steps/run-tests-windows.yml

@@ -25,7 +25,7 @@ steps:
     Set-Acl "R:\Temp" $acl
   displayName: Set RAMDisk Permissions
 
-- bash: pip install --upgrade setuptools tox
+- bash: pip install --upgrade setuptools tox virtualenv
   displayName: Install Tox
 
 - script: tox -e py -- -m unit -n auto --junit-xml=junit/unit-test.xml