Add Python 3.7 to the list of interpreters for virtual environment UI tests.
Add Python 3.7 to the list of interpreters for virtual environment UI tests.

huguesv/PTVS,huguesv/PTVS,huguesv/PTVS,zooba/PTVS,zooba/PTVS,Microsoft/PTVS,int19h/PTVS,Microsoft/PTVS,zooba/PTVS,int19h/PTVS,int19h/PTVS,huguesv/PTVS,zooba/PTVS,huguesv/PTVS,zooba/PTVS,int19h/PTVS,int19h/PTVS,Microsoft/PTVS,int19h/PTVS,Microsoft/PTVS,Microsoft/PTVS,Microsoft/PTVS,zooba/PTVS,huguesv/PTVS