`basic_install.sh`: `pyvenv` was removed from Python3.8

[OrRosenblatt]

pyvenv was removed from Python3.8, but the basic_install.sh script is still relying on this library. Can probably be replaced with python3 -m venv for newer Python versions.

[rilozos]

I have addressed the issue in my pull request: pyvenv depricated in 3.8+, updated to python3 -m venv