virtualenv vs venv in installation instructions

[KonradHoeffner]

$ python3 -m virtualenv venv
/usr/bin/python3: No module named virtualenv

Either add pip install virtualenv to the docs or use -m venv instead if that is enough, as it is shipped by default with Python3.3+, see https://stackoverflow.com/questions/41573587/what-is-the-difference-between-venv-pyvenv-pyenv-virtualenv-virtualenvwrappe.

Part of the JOSS review https://github.com/openjournals/joss-reviews/issues/5017.

[Ayuei]

Hi, thanks for the issue. Looking at the differences, virtualenv isn't required over venv.

Fixed via 0248b43ffe58c0225ad3c644b0d4b34c584be37f