Fix CI pipeline and remove version pinning in setup script

Remove pinning of python version in setup script so that a more current version than 3.7 can be used when available on the system. Use python 3.9 in CI pipeline and make sure the correct version of virtualenv is installed. Previously, either no candidate for virtualenv could be found or 3.9 was installed but setup script then tried to create a 3.7 virtualenv.