Upgrade virtualenv in dev/tests environments (#7287)
Upgrade virtualenv in dev/tests environments (#7287)

AppVeyor recently upgrade the Python 3.7.x installed in their VM to 3.7.4. However, virtualenv 16.6.1 is broken on that specific version of Python for Windows.

This PR upgrade virtualenv installed for a dev/test environment from 16.6.1 to 16.6.2 in order to fix this issue, and repair the CI jobs execute by AppVeyor on PRs.
stweil/letsencrypt,stweil/letsencrypt,lmcro/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt,letsencrypt/letsencrypt